Change filename to match first use of file

Step 11 online (appears to be the second step 8 in the .md file) states: "Build another HTML file called `thanks.html`.  It should look like:".  However when the same file is referenced later, it is referred to as thank_you.html.  I changed both occurrences of thank_you.html to thanks.html
Merge pull request #248 from donaldali/patch-14

Change filename to match first use of file