Fix Mockito docs for -javaagent flag with Gradle - #3866

Fixes #3864

These examples now match the already-linked Gradle docs:
https://docs.gradle.org/current/userguide/caching_java_projects.html#dealing_with_file_paths

Specifically:

• use jvmArgumentProviders API in Kotlin example (previous example was broken)
• use sub-class of CommandLineArgumentProvider
• fix @ escaping (This was clobbering the entire remainder of the javadoc)

Checklist

• Read the contributing guide
• PR should be motivated, i.e. what does it fix, why, and if relevant how
• If possible / relevant include an example in the description, that could help all readers
  including project members to get a better picture of the change
• Avoid other runtime dependencies
• Meaningful commit history ; intention is important please rebase your commit history so that each
  commit is meaningful and help the people that will explore a change in 2 years
• The pull request follows coding style (run ./gradlew spotlessApply for auto-formatting)
• Mention Fixes #<issue number> in the description if relevant
• At least one commit should end with Fixes #<issue number> if relevant