docs(auth): add missing import statement for google sign-in

[palcodes]

The google_sign_in package from pub.dev/packages/google_sign_in is being used but no import statement is mentioned for the same in the README. And it is mandatory to import the google_sign_in package for creating an instance of GoogleSignIn i.e. working with Google Authentication.

Description

• This PR simply places an import statement for the package - google_sign_in.
• Beginners might come across the documentation and be confused as to what else is needed to import for Google auth.
• This slightly makes it insightful for the document reader that he/she needs to import the google_sign_in package from pub.dev [link provided above].
• In fact the firebase_auth documentation for auth does mention using the plugin to implement Google authentication, but the section Use The Plugin is a bit misleading for a newcomer.

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[palcodes]

@googlebot I signed it!

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CLAs look good, thanks!

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[Salakar]

Thank you :)

[palcodes]

Hey, @Salakar thanks for approving 😊
This change is minor but does help in providing a context to the reader that he/she needs to use the specified package, considering the snippet of code just before and after the import line, requires google_sign_in to be imported.
Looking forward to making more contributions as I learn.