[firebase_admob] Deleted a line from README.md about not supporting native ads

[dshukertjr]

Description

As of version 0.9.3 of the firebase_admob plugin, native ads are supported (at least for the basic part), so took out a line that says "There's no support for native ads." from README.md.

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[bparrishMines]

LGTM

Thanks for the contribution!