Doc Change only: change from sdk 2.0 to 3.0

[nneuberger1]

change from 2.0 to 3.0. I was using square_in_app_payments: ^1.7.1 an…d caused incompatibility and noticed upgrading will statisfy the build failure between the two sdks.

Summary

Documentation only. I'm using both square_in_app_payments and square_reader_sdk in the same project and I noticed a dependency failure.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Nick Neuberger seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}