fix potentially confusing invocation in README.md
fix potentially confusing invocation in README.md

The current phrasing can imply that the thunk is called with `dispatch` and the **invocation** of `getState`, since it says `getState()` but not `dispatch()`. This PR removes the invocation parens from `getState`.
gaearon/redux-thunk,reduxjs/redux-thunk,gaearon/redux-thunk,davnicwil/redux-thunk,reduxjs/redux-thunk,davnicwil/redux-thunk