variances-page: use T and S instead of A and B

[Philippus]

like in the description and some styling fixes

mentioned in the comments of http://docs.scala-lang.org/tutorials/tour/variances.html

[SethTisue]

someone was telling me recently that A and B are more idiomatic in Scala, and perhaps we should try to be consistent on this tour-wide. but regardless, this is an improvement. thank you!

[som-snytt]

I also learned early to use A and B and find anything else jarring except CC in collections, or maybe Elem.

I haven't heard why that's idiomatic but not f(x, y, z). With named and partial application of type params, the mores would change.

What the neutral A expresses is, as a List[A], I don't care what I'm a list of. But if I'm a Map[A, B], I might prefer Map[K, V] or similar.