Compound Types doc page should point to Class Composition Mixin page for refinement explanation

[iserraller]

Currently the last line of the Compound Types page in the Tour of Scala reads:

An example for the use of refinements is given on the page about abstract types.

However refinements are not mentioned at all in the Abstract Types page. The concept is introduced, although the word refinements is not used, in the Class Composition with Mixins page. So maybe that last line should instead be:

An example for the use of refinements is given on the page about class composition with mixins.

[SethTisue]

good catch — yes, a pull request on this would be welcome