Fix typo in fd.c

The frontend version of walkdir() is defined in file_utils.c, and not
initdb.c.

Author: Sehrope Sarkuni
Discussion: https://postgr.es/m/CAH7T-artawnBt4=KODNCD8Mt2ZX4CCjJT8c=_=950xjutcRZ4Q@mail.gmail.com

src/backend/storage/file/fd.c

@@ -3208,7 +3208,8 @@ SyncDataDirectory(void)
  *
  * Errors are reported at level elevel, which might be ERROR or less.
  *
- * See also walkdir in initdb.c, which is a frontend version of this logic.
+ * See also walkdir in file_utils.c, which is a frontend version of this
+ * logic.
  */
 static void
 walkdir(const char *path,