updating docs from 'useSignInCheck' to 'useSigninCheck'

[thomasmburke]

Ran into import errors when following the docs because of this subtle lowercase i.

I think overall it would be worth having this camel case exception be consistent with signInCheckResult, but not worth introducing a breaking change in my opinion, so just looking to update the docs and references in this repo.

[jhuleatt]

Thanks @thomasmburke!

[jhuleatt]

fixed in this commit: https://github.com/FirebaseExtended/reactfire/commit/414201e88bd6f6701998993f6203e4d49773147b

[thomasmburke]

Sorry for missing this!