Remove obsolete diskbuffer setting from example config file

The configuration document has an example config file that refers
to the diskbuffer setting, which was removed in Ia82d056.

Update the document to remove the obsolete setting.

Change-Id: I1489b483831b992587f215b320530df8eaa26d54
diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 588a2fc..0a5628a 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -19,9 +19,6 @@
 
 [cache]
   directory = /var/cache/gerrit2
-
-[cache "diff"]
-  diskbuffer = 10 m
 ----
 
 [[accounts]]Section accounts