docs(mmkv): fix plugin name and install step

[frankcalise]

Please verify the following:

• [ ] yarn build-and-test:local passes
• [ ] I have added tests for any new features, if relevant
• [ ] README.md (or relevant documentation) has been updated with your changes

Describe your PR

Updates reactotron-react-native-mmkv plugin docs with install step and a typo fix