Replace reference to "xlog-method" with "wal-method" in error message.

Commit 62e8b38 renamed "--xlog-method" option for pg_basebackup to
"--wal-method", but forgot to update the error message mentioning that option.

src/bin/pg_basebackup/pg_basebackup.c

@@ -2170,7 +2170,7 @@ main(int argc, char **argv)
 				else
 				{
 					fprintf(stderr,
-							_("%s: invalid xlog-method option \"%s\", must be \"fetch\", \"stream\" or \"none\"\n"),
+							_("%s: invalid wal-method option \"%s\", must be \"fetch\", \"stream\" or \"none\"\n"),
 							progname, optarg);
 					exit(1);
 				}