trivial: fix mongos header from using isolation

Author: kay-kim

source/core/sharded-cluster-query-router.txt

@@ -165,8 +165,8 @@ returned document does not include the ``isdbgrid`` string.
 .. _sharding-shard-key-query-isolation:
 .. _sharding-query-isolation:
 
-Query Isolation
----------------
+Targeted Operations vs. Broadcast Operations
+--------------------------------------------
 
 Generally, the fastest queries in a sharded environment are those that
 :binary:`~bin.mongos` route to a single shard, using the :term:`shard key` and the