C comment: fix typo, log -> lag

Reported-by: atorikoshi

Discussion: https://postgr.es/m/b61f2ab9-c0e0-d33d-ce3f-42a228025681@lab.ntt.co.jp

Author: atorikoshi

src/backend/replication/walsender.c

@@ -1245,7 +1245,7 @@ WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
 /*
  * LogicalDecodingContext 'update_progress' callback.
  *
- * Write the current position to the log tracker (see XLogSendPhysical).
+ * Write the current position to the lag tracker (see XLogSendPhysical).
  */
 static void
 WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid)