Change "checkpoint starting" message to use "wal"

This catches up with the recent renaming of all user-facing mentions
of "xlog" to "wal".

Discussion: https://www.postgresql.org/message-id/flat/20181129084708.GA9562%40msg.credativ.de

src/backend/access/transam/xlog.c

@@ -8383,7 +8383,7 @@ LogCheckpointStart(int flags, bool restartpoint)
 		 (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
 		 (flags & CHECKPOINT_FORCE) ? " force" : "",
 		 (flags & CHECKPOINT_WAIT) ? " wait" : "",
-		 (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
+		 (flags & CHECKPOINT_CAUSE_XLOG) ? " wal" : "",
 		 (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "",
 		 (flags & CHECKPOINT_FLUSH_ALL) ? " flush-all" : "");
 }