README.md typos and fixes

Just a couple fixes for the README:
* Update the maven release version `2.0.0` => `2.0.2` to reflect latest release
* As described in [this commit](https://github.com/twitter/hbc/commit/6f0c481daafb253209832bde0241e0d0f41c362c): StreamingEndpoint does not have methods `endpoint.followings(followings);` and `endpoint.trackTerms(terms);`. Should probably be `StatusesFilterEndpoint`.
* Fix variable name for endpoint to match example further down.
* Fix variable name for client to match previous code.
* Fix incomplete sentence describing enumerated endpoints.
* Fix typo "could be change"