do not spellcheck vendored scripts (#55020)

Summary:
While running `yarn spellcheck` I have spotted that scripts located in `/vendor` directory are checked, which lead to a lot of warnings in the output.

This PR adds an exclusion for the `/vendor` path in the spellcheck script which leads to way smaller warnings set and saves ~10s of runtime on my machine.

## Changelog:

[INTERNAL] [FIXED] - Add `/vendor` to excluded paths for spellcheck script.

Pull Request resolved: https://github.com/facebook/react-native/pull/55020

Test Plan: Running `yarn spellcheck` locally.

Reviewed By: cipolleschi

Differential Revision: D90104078

Pulled By: cortinico

fbshipit-source-id: 9c16a71616a13db1c217b917a498ff8f6a462799