Fix incorrect function name in comments

Update comments to reference WaitForLSN() instead of the outdated
WaitForLSNReplay() function name.

Discussion: https://postgr.es/m/CABPTF7UieOYbOgH3EnQCasaqcT1T4N6V2wammwrWCohQTnD_Lw%40mail.gmail.com
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>

src/backend/commands/wait.c

@@ -143,8 +143,7 @@ ExecWaitStmt(ParseState *pstate, WaitStmt *stmt, DestReceiver *dest)
 	waitLSNResult = WaitForLSN(WAIT_LSN_TYPE_REPLAY, lsn, timeout);
 
 	/*
-	 * Process the result of WaitForLSNReplay().  Throw appropriate error if
-	 * needed.
+	 * Process the result of WaitForLSN().  Throw appropriate error if needed.
 	 */
 	switch (waitLSNResult)
 	{

src/include/access/xlogwait.h

@@ -20,7 +20,7 @@
 #include "tcop/dest.h"
 
 /*
- * Result statuses for WaitForLSNReplay().
+ * Result statuses for WaitForLSN().
  */
 typedef enum
 {