Remove genericity from pattern matching example.
Remove genericity from pattern matching example.

This simplifies the example, as the reader is not
distracted with the type parameterization (which is
non-obvious due to covariance).

boldradius/scala-lang,boldradius/scala-lang,jeantil/scala-lang,vl4ds/scala-lang,vl4ds/scala-lang,andy1138/scala-lang,jeantil/scala-lang,boldradius/scala-lang,jeantil/scala-lang,vl4ds/scala-lang,andy1138/scala-lang,andy1138/scala-lang,andy1138/scala-lang,jeantil/scala-lang,boldradius/scala-lang,vl4ds/scala-lang