fixing a typo in README.md

[deepueg]

Hey there! So you want to contribute to FirebaseUI? Before you file this pull request, follow these steps:

• Read the contribution guidelines.
• If this has been discussed in an issue, make sure to mention the issue number here. If not, go file an issue about this to make sure this is a desirable change.
• If this is a new feature please co-ordinate with someone on FirebaseUI-iOS to make sure that we can implement this on both platforms and maintain feature parity.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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• If you signed the CLA as a corporation, please let us know the company's name.

[deepueg]

I signed it.

[samtstern]

LGTM, thanks!