Document the update highlight.js in Gerrit v3.1.13

Add the highlight.js dependency upgrade previously
missed in the Gerrit v3.1.13 release notes.

Change-Id: I223ec78feb3589768ed01c68be21555a82e64e7a
diff --git a/pages/site/releases/3.1.md b/pages/site/releases/3.1.md
index 2a71c5b..d55acd5 100644
--- a/pages/site/releases/3.1.md
+++ b/pages/site/releases/3.1.md
@@ -358,6 +358,10 @@
   * Development guidelines: mention that Optional in arguments is discouraged
    (use @Nullable instead) but may be used as return type.
 
+* Dependency Updates
+
+  * Update highlight.js to 10.6.0.
+
 ### 3.1.12
 
 * New Features