Update mtr help

- Based on patch: d6a983351c5a454bd0cb113852f
- Update combination example for 10.2 (commit 2a3fe45d added change
for 10.3+)
```
==============================================================================

TEST                                      RESULT   TIME (ms) or COMMENT
--------------------------------------------------------------------------

worker[1] Using MTR_BUILD_THREAD 300, with reserved ports 16000..16019
rpl.rpl_invoked_features 'innodb,mix'    [ pass ]   1677
rpl.rpl_invoked_features 'innodb,row'    [ pass ]   3516
rpl.rpl_invoked_features 'innodb,stmt'   [ pass ]   1609
--------------------------------------------------------------------------
```
- `gdb` option will be added during the merge

mysql-test/mysql-test-run.pl

@@ -6418,7 +6418,7 @@ Examples:
 
 alias
 main.alias              'main' is the name of the suite for the 't' directory.
-rpl.rpl_invoked_features,mix,xtradb_plugin
+rpl.rpl_invoked_features,mix,innodb
 suite/rpl/t/rpl.rpl_invoked_features
 
 Options to control what engine/variation to run: