MDEV-16944 postfix. Fix a typo

mysql-test/main/mysql_client_test_nonblock.test

@@ -19,7 +19,7 @@ call mtr.add_suppression(" IP address .* could not be resolved");
 # server or run mysql-test-run --debug mysql_client_test and check
 # var/log/mysql_client_test.trace
 
----write_line "$MYSQL_CLIENT_TEST --non-blocking-api" $MYSQLTEST_VARDIR/log/mysql_client_test.out.log
+--write_line "$MYSQL_CLIENT_TEST --non-blocking-api" $MYSQLTEST_VARDIR/log/mysql_client_test.out.log
 --exec $MYSQL_CLIENT_TEST --non-blocking-api --getopt-ll-test=25600M >> $MYSQLTEST_VARDIR/log/mysql_client_test.out.log 2>&1
 
 # End of 4.1 tests