Fix typo

In `for inner classes `val mm2 = im.reflectClass(<module symbol>)``, `<module symbol>` should read `<class symbol>`.

The same error is also in https://github.com/scala/scala/blob/master/src/reflect/scala/reflect/api/Mirrors.scala#L156, for which I'm sending a separate pull request.
Merge pull request #284 from Blaisorblade/patch-1

Fix typo