Fix thinko in comment

This comment has been wrong since its introduction in commit
2c03216d.

Author: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/CAD21AoAzz6qipFJBbGEaHmyWxvvNDp8httbwLR9tUQWaTjUs2Q@mail.gmail.com

src/backend/access/transam/xlogreader.c

@@ -1545,7 +1545,7 @@ XLogRecGetBlockData(XLogReaderState *record, uint8 block_id, Size *len)
 /*
  * Restore a full-page image from a backup block attached to an XLOG record.
  *
- * Returns the buffer number containing the page.
+ * Returns true if a full-page image is restored.
  */
 bool
 RestoreBlockImage(XLogReaderState *record, uint8 block_id, char *page)