Fix link to the license

Inspired by 1526e8b (Fix link to “add GUI” section, 2025-02-16), I had
a look around whether there were more stale references to the `main`
branch of the `git/git-scm.com` repository (which switched to `gh-pages`
as the default branch), and found only this one.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

content/site/_index.html

@@ -24,7 +24,7 @@ <h2>Open Source</h2>
     repository at GitHub. That includes all pages except the book and
     manpages, along with the general design of the site. This content
     is available under the
-    <a href="https://github.com/git/git-scm.com/blob/main/MIT-LICENSE.txt">MIT license</a>.
+    <a href="https://github.com/git/git-scm.com/tree/gh-pages?tab=MIT-1-ov-file#readme">MIT license</a>.
     Bug reports and suggestions may be made in that repository.
 
     <li>Content from the ProGit book is imported from the