Fix broken "Contributing" link

Contributing information has moved from index.html to
dev-community.html under the headline "How to contribute".

See:
https://gerrit-review.googlesource.com/c/gerrit/+/221952

Change-Id: I14302f6d827f05ca73c2f992dde73fa4e8d18f74
diff --git a/_data/topnav.yml b/_data/topnav.yml
index 7811712..de9b0df 100644
--- a/_data/topnav.yml
+++ b/_data/topnav.yml
@@ -20,7 +20,7 @@
         - title: Members
           url: /members.html
         - title: Contributing
-          external_url: https://gerrit-documentation.storage.googleapis.com/Documentation/3.1.4/index.html#_developer
+          external_url: https://gerrit-documentation.storage.googleapis.com/Documentation/3.1.4/dev-community.html#how-to-contribute
         - title: Kudos
           url: /kudos.html
         - title: User Stories