/community: use internal link for SubmittingPatches

"SubmittingPatches" is built on the website
since commmit ea0e16d (docs: include SubmittingPatches, 2018-04-16),
so link to it instead of linking to the raw document on GitHub.

app/views/community/index.html.erb

@@ -78,5 +78,5 @@
  <h2> Contributing to Git </h2>
 
  <p>
- The <a href="https://github.com/git/git/tree/master/Documentation">Documentation directory</a> in the Git source code has several files of interest to developers who are looking to help contribute. After reading the <a href="https://github.com/git/git/blob/master/Documentation/CodingGuidelines">coding guidelines</a>, you can learn <a href="https://github.com/git/git/blob/master/Documentation/SubmittingPatches">how to submit patches</a>. For those looking to get more deeply involved, there is a <a href="https://github.com/git/git/blob/master/Documentation/howto/maintain-git.txt">howto for Git maintainers</a>.
+ The <a href="https://github.com/git/git/tree/master/Documentation">Documentation directory</a> in the Git source code has several files of interest to developers who are looking to help contribute. After reading the <a href="https://github.com/git/git/blob/master/Documentation/CodingGuidelines">coding guidelines</a>, you can learn <a href="/docs/SubmittingPatches">how to submit patches</a>. For those looking to get more deeply involved, there is a <a href="https://github.com/git/git/blob/master/Documentation/howto/maintain-git.txt">howto for Git maintainers</a>.
  </p>