Fix typo in explanation of adapter.js shim

[iAmmar7]

In Step 5 of the documentation, there is a grammatical error in the explanation of the adapter.js shim. The sentence "In this step, we've linked to the most recent version of adapter.js, which is fine for a codelab but not may not be right for a production app" contains a repetition of the word "not" which makes the sentence incorrect.

Suggested revision:
"In this step, we've linked to the most recent version of adapter.js, which is fine for a codelab but may not be right for a production app"

Please consider updating the documentation accordingly. Thank you.