Change curl statement in README.md

For reference:

    $ curl -iw '\n' http://tinyurl.com/paulp-sbt
    HTTP/1.1 301 Moved Permanently
    X-Powered-By: PHP/5.4.27
    Location: https://github.com/paulp/sbt-extras/raw/master/sbt
    X-tiny: cache 0.010701179504395
    Content-Type: text/html
    Content-Length: 0
    Server: TinyURL/1.6
    Date: Tue, 15 Jul 2014 06:29:02 GMT
    Connection: keep-alive
    Set-Cookie: tinyUUID=3c4ca32a077a4e951a99e048; expires=Wed, 15-Jul-2015
    06:29:01 GMT; path=/; domain=.tinyurl.com

    $ curl -iw '\n' https://github.com/paulp/sbt-extras/raw/master/sbt
    HTTP/1.1 302 Found
    Server: GitHub.com
    Date: Tue, 15 Jul 2014 06:29:08 GMT
    Content-Type: text/html; charset=utf-8
    Status: 302 Found
    Cache-Control: no-cache
    X-XSS-Protection: 1; mode=block
    X-Frame-Options: deny
    Content-Security-Policy: default-src *; script-src assets-cdn.github.com
    www.google-analytics.com collector-cdn.github.com; object-src
    assets-cdn.github.com; style-src 'self' 'unsafe-inline' 'unsafe-eval'
    assets-cdn.github.com; img-src 'self' data: assets-cdn.github.com
    identicons.github.com www.google-analytics.com collector.githubapp.com
    *.githubusercontent.com *.gravatar.com *.wp.com; media-src 'none';
    frame-src 'self' render.githubusercontent.com www.youtube.com
    assets.braintreegateway.com; font-src assets-cdn.github.com; connect-src
    'self' ghconduit.com:25035 live.github.com uploads.github.com
    s3.amazonaws.com
    Vary: X-PJAX
    X-RateLimit-Limit: 100
    X-RateLimit-Remaining: 100
    Access-Control-Allow-Origin: https://render.githubusercontent.com
    Location: https://raw.githubusercontent.com/paulp/sbt-extras/master/sbt
    Content-Length: 127
    Set-Cookie:
    _gh_sess=eyJzZXNzaW9uX2lkIjoiZDI5NDE2NzBjZjJjZWJiYWIyN2FmYmJkYWNiNzk4MjciLCJzcHlfcmVwbyI6InBhdWxwL3NidC1leHRyYXMiLCJzcHlfcmVwb19hdCI6MTQwNTQwNTc0OH0%3D--e6ca04c9ea85b880cf2dae3a59bb2a950e8a5a07;
    path=/; secure; HttpOnly
    X-GitHub-Request-Id: 4E951A99:1160:7E26AAA:53C4CA34
    Strict-Transport-Security: max-age=31536000; includeSubdomains
    X-Content-Type-Options: nosniff
    Vary: Accept-Encoding
    X-Served-By: bc4c952d089501afbfc8f7ff525da31c

    <html><body>You are being <a
    href="https://raw.githubusercontent.com/paulp/sbt-extras/master/sbt">redirected</a>.</body></html>