AggregateQuerySnapshot.java: minor change to javadocs for getCount()

Author: dconeybe

firebase-firestore/src/main/java/com/google/firebase/firestore/AggregateQuerySnapshot.java

@@ -43,7 +43,7 @@ public AggregateQuery getQuery() {
     return query;
   }
 
-  /** Returns the number of documents in the result set of the query. */
+  /** Returns the number of documents in the result set of the underlying query. */
   public long getCount() {
     return count;
   }