Typo "gradle.build" instead of "build.gradle"

The [Add Play Core library to your app](https://developer.android.com/codelabs/on-demand-dynamic-delivery#2) step includes the following sentence:
"So that the complete dependency, in the main module gradle.build file, became:"

gradle.build should be build.gradle

Open the linked “Add Play Core library to your app” codelab and locate the sentence in the named step. Replace “gradle.build” with “build.gradle,” then verify the rendered codelab shows the corrected filename.

Written by the indexing model from the issue text.