version mistake in build.gradle file

At https://developer.android.com/codelabs/android-proto-datastore#4
```
protobuf {
protoc {
artifact = "com.google.protobuf:protoc:21.7"
}
```
Should be

```
protobuf {
protoc {
artifact = "com.google.protobuf:protoc:3.21.7"
}
```

Open the build.gradle file used in the Android Proto DataStore codelab at the referenced step and inspect the protobuf protoc artifact version. Done means correcting the version from 21.7 to 3.21.7 and confirming the codelab shows the corrected dependency.

Written by the indexing model from the issue text.