fix snapshot gradle implementation typo

[frapeti]

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

• Read the contribution guidelines.
• Run ./gradlew check to ensure the Travis build passes.
• 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 (if not, look below for help). 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 (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

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[samtstern]

@frapeti thanks for this! If you don't want to sign the CLA for a one-line change just let me know and I will make this change for you.

Worth noting that signing the CLA is a one-time thing for all Google repos though.

[frapeti]

@samtstern just signed the CLA 🙂

[googlebot]

CLAs look good, thanks!

[samtstern]

Uhhh, seems like I just locked myself out of the house:

[samtstern]

@frapeti thanks for your help and patience!