Fix some issues with the mockito README.md
Fix some issues with the mockito README.md

* The example uses code-generation but doesn't use nullable types. (I
  suppose this might be technically okay, but I'm skeptical that
  anyone would use code-generation without null-safety.)

* Use `dart run` intead of `pub run` to perform code generation.

* Remove references to unstubbed methods returning null by default.

* Fix an example that compared a `List` element to a `List`.

* Fix a broken link.

* Consistently use the American English spelling of "behavior".

dart-lang/mockito