Bump doc ETag generation to 4

With the fixes for relative links we want to invalidate current cached
pages so browsers can view the corrected links.

Change-Id: I2714e4109524b08f2faa0a4116219ff3f4105ec9

gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java
index bdb28a4..1ba14a8 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java
@@ -69,7 +69,7 @@
   // Generation of ETag logic. Bump this only if DocServlet logic changes
   // significantly enough to impact cached pages. Soy template and source
   // files are automatically hashed as part of the ETag.
-  private static final int ETAG_GEN = 3;
+  private static final int ETAG_GEN = 4;
 
   public DocServlet(GitilesAccess.Factory accessFactory, Renderer renderer) {
     super(renderer, accessFactory);