make link relative

Otherwise it doesn’t work locally.

index.html

@@ -990,7 +990,7 @@ <h3>Create MD5 checksums</h3>
             <dt>-an</dt><dd>ignores the audio stream (audio no)</dd>
             <dt><i>output_file</i></dt><dd>path, name and extension of the output file</dd>
           </dl>
-          <p>You may verify an MD5 checksum file created this way by using a <a href="https://github.com/amiaopensource/ffmprovisr/blob/gh-pages/check_framemd5.sh" target="_blank">Bash script</a>.</p>
+          <p>You may verify an MD5 checksum file created this way by using a <a href="check_framemd5.sh" target="_blank">Bash script</a>.</p>
           <p class="link"></p>
         </div>
       </div>