changed link to point directly to Dockerfile

The original link to build the docker image gives this error:

>Step 0 : <!DOCTYPE html>
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>Step 1 : <html class="   ">
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>Step 3 : <meta charset='utf-8'>
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>Step 5 : <title>kasimon/docker-heartbleed · GitHub</title>
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>\# Skipping unknown instruction <LINK
>Step 7 : <link rel="fluid-icon" href="https://github.com/fluidicon.png" title="GitHub" />
>\# Skipping unknown instruction <LINK
>Step 8 : <link rel="apple-touch-icon" sizes="57x57" href="/apple-touch-icon-114.png" />
>\# Skipping unknown instruction <LINK
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>\# Skipping unknown instruction <LINK
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>\# Skipping unknown instruction <LINK
>Step 11 : <link rel="apple-touch-icon" sizes="144x144" href="/apple-touch-icon-144.png" />
>\# Skipping unknown instruction <LINK
>Step 12 : <meta property="fb:app_id" content="1401488693436528"/>
>\# Skipping unknown instruction <META
>Step 13 : <meta content="@github" name="twitter:site" /><meta content="summary" name="twitter:card" /><meta content="kasimon/docker-heartbleed" name="twitter:title" /><meta content="docker-heartbleed - Dockerfile to install and run heartbleed tool" name="twitter:description" /><meta content="https://avatars2.githubusercontent.com/u/78341?s=400" name="twitter:image:src" />
>\# Skipping unknown instruction <META
>Step 14 : <meta content="GitHub" property="og:site_name" /><meta content="object" property="og:type" /><meta content="https://avatars2.githubusercontent.com/u/78341?s=400" property="og:image" /><meta content="kasimon/docker-heartbleed" property="og:title" /><meta content="https://github.com/kasimon/docker-heartbleed" property="og:url" /><meta content="docker-heartbleed - Dockerfile to install and run heartbleed tool" property="og:description" />
>\# Skipping unknown instruction <META
>Step 15 : <link rel="assets" href="https://github.global.ssl.fastly.net/">
>\# Skipping unknown instruction <LINK
>Step 16 : <link rel="conduit-xhr" href="https://ghconduit.com:25035/">
>\# Skipping unknown instruction <LINK
>Step 17 : <link rel="xhr-socket" href="/_sockets" />
>\# Skipping unknown instruction <LINK
>Step 18 : <meta name="msapplication-TileImage" content="/windows-tile.png" />
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>Step 20 : <meta name="selected-link" value="repo_source" data-pjax-transient />
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>\# Skipping unknown instruction <META
>Step 22 : <link rel="icon" type="image/x-icon" href="https://github.global.ssl.fastly.net/favicon.ico" />
>\# Skipping unknown instruction <LINK
>Step 23 : <meta content="authenticity_token" name="csrf-param" />
>\# Skipping unknown instruction <META
>Step 24 : <meta content="jWaV2UE7GmvOmEur+BPEiBEqKE6iwqzd3+tWss39y+g=" name="csrf-token" />
>\# Skipping unknown instruction <META
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>\# Skipping unknown instruction <LINK
>Step 26 : <link href="https://github.global.ssl.fastly.net/assets/github2-b564d9e99bd5747d2d0901c61c403e758a5e5ef1.css" media="all" rel="stylesheet" type="text/css" />
>\# Skipping unknown instruction <LINK
>Step 27 : <script crossorigin="anonymous" src="https://github.global.ssl.fastly.net/assets/frameworks-8abcd7ea143a104f306837078bc3832700c88b3a.js" type="text/javascript"></script>
>\# Skipping unknown instruction <SCRIPT
>Step 28 : <script async="async" crossorigin="anonymous" src="https://github.global.ssl.fastly.net/assets/github-984ce95f5187e6d78b77f2f469b90465c7fd9a1d.js" type="text/javascript"></script>
>\# Skipping unknown instruction <SCRIPT
>Step 29 : <meta http-equiv="x-pjax-version" content="414fde84db740e83c88e09105b62dd03">
>\# Skipping unknown instruction <META
>Step 30 : <meta name="description" content="docker-heartbleed - Dockerfile to install and run heartbleed tool" />
>\# Skipping unknown instruction <META
>Step 31 : <meta content="78341" name="octolytics-dimension-user_id" /><meta content="kasimon" name="octolytics-dimension-user_login" /><meta content="18594083" name="octolytics-dimension-repository_id" /><meta content="kasimon/docker-heartbleed" name="octolytics-dimension-repository_nwo" /><meta content="true" name="octolytics-dimension-repository_public" /><meta content="false" name="octolytics-dimension-repository_is_fork" /><meta content="18594083" name="octolytics-dimension-repository_network_root_id" /><meta content="kasimon/docker-heartbleed" name="octolytics-dimension-repository_network_root_nwo" />
>\# Skipping unknown instruction <META
>Step 32 : <link href="https://github.com/kasimon/docker-heartbleed/commits/master.atom" rel="alternate" title="Recent Commits to docker-heartbleed:master" type="application/atom+xml" />
>\# Skipping unknown instruction <LINK
>Step 33 : </head>
>2014/04/09 18:24:24 Invalid Dockerfile format