Fix comment that was missing a word.

Ted Yu

Discussion: http://postgr.es/m/CALte62wkFB05=RTWf7BL_6MfWs2=DY=ai-K7LWn_+0TJUuPJ2w@mail.gmail.com

src/include/utils/backend_status.h

@@ -273,8 +273,8 @@ typedef struct LocalPgBackendStatus
 	int	backend_subxact_count;
 
 	/*
-	 * The number of subtransactions in the current session exceeded the cached
-	 * subtransaction limit.
+	 * The number of subtransactions in the current session which exceeded the
+	 * cached subtransaction limit.
 	 */
 	bool backend_subxact_overflowed;
 } LocalPgBackendStatus;