minor: fixing broken glossary link

Author: Sam Kleinman

source/administration/indexes.txt

@@ -266,8 +266,8 @@ building process blocks all replication and read operations on the
 secondaries while they build the index.
 
 Secondaries will begin building indexes *after* the
-:term:`primary` finishes building the index. In :term:`shard clusters`,
-the :program:`mongos` will send :func:`ensureIndex()
+:term:`primary` finishes building the index. In :term:`shard clusters
+<shard cluster>`, the :program:`mongos` will send :func:`ensureIndex()
 <db.collection.ensureIndex()>` to the primary members of the replica
 set for each shard, which then replicate to the secondaries after the
 primary finishes building the index.