Fix comment in xlog.c.

This commit fixes the comment about SharedHotStandbyActive variable.
The comment was apparently copy-and-pasted.

Author: Atsushi Torikoshi
Discussion: https://postgr.es/m/CACZ0uYEjpqZB9wN2Rwc_RMvDybyYqdbkPuDr1NyxJg4f9yGfMw@mail.gmail.com

src/backend/access/transam/xlog.c

@@ -649,8 +649,8 @@ typedef struct XLogCtlData
 	bool		SharedRecoveryInProgress;
 
 	/*
-	 * SharedHotStandbyActive indicates if we're still in crash or archive
-	 * recovery.  Protected by info_lck.
+	 * SharedHotStandbyActive indicates if we allow hot standby queries to be
+	 * run.  Protected by info_lck.
 	 */
 	bool		SharedHotStandbyActive;