Mention documentation updates all caps for 2.15.21

Update the release notes for Gerrit v2.15.21, making consistent the
capitalisation of the 'Documentation Updates' title.

Change-Id: Ic5b0c743f224b6a1848888ab71da1e3d253cd248
diff --git a/pages/site/releases/2.15.md b/pages/site/releases/2.15.md
index 445edfe..5d2868e 100644
--- a/pages/site/releases/2.15.md
+++ b/pages/site/releases/2.15.md
@@ -303,7 +303,7 @@
 `(prohibited by Gerrit)` has been replaced with a detailed message describing
 the exact permissions required.
 
-### Documentation updates
+### Documentation Updates
 
 With this release, we have updated several sections of the documentation to
 make it easier for users to find the information they need. These updates
@@ -492,7 +492,7 @@
     when the target repository is `All-Users`, where nobody can be authorized
     to skip the ACLs evaluation anyway.
 
-* Bug Fixes
+* Documentation Updates
 
   * [Issue 12869](https://bugs.chromium.org/p/gerrit/issues/detail?id=12869):
     Add 'Move a Change' section
@@ -879,7 +879,7 @@
 
   * Upgrade polymer-resin to 2.0.1.
 
-* Documentation updates
+* Documentation Updates
 
   * [Issue 10897](https://bugs.chromium.org/p/gerrit/issues/detail?id=10897):
   Update links to Google individual and corporate CLA pages.