Update WritingTests.md (#2654)
Update WritingTests.md (#2654)

Example for middleware testing needs to destructure the `store` param that is being passed in to `invoke()`. Currently, the example shows `invoke((dispatch, getState) => { ... })` on LINE 456, but it should read as `invoke(({ dispatch, getState }) => { ... })`.
CompuIves/redux,gaearon/redux,CompuIves/redux,gaearon/redux