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.

Author: Blaisorblade

overviews/reflection/environment-universes-mirrors.md

@@ -138,7 +138,7 @@ A `FieldMirror` is used for getting/setting instance fields (like methods, Scala
     scala> fmY.get
     res2: Any = 4
 
-A `ClassMirror` is used for creating invoker mirrors for constructors. Entry points: for static classes `val cm1 = m.reflectClass(<class symbol>)`, for inner classes `val mm2 = im.reflectClass(<module symbol>)`. Example:
+A `ClassMirror` is used for creating invoker mirrors for constructors. Entry points: for static classes `val cm1 = m.reflectClass(<class symbol>)`, for inner classes `val mm2 = im.reflectClass(<class symbol>)`. Example:
 
     scala> case class C(x: Int)
     defined class C