2.x: The RxJavaPlugins JavaDocs HTML should be auto-cleaned up

[akarnokd]

Some method entries of the RxJavaPlugins JavaDocs HTML looks misaligned, just like some other components used to:

The fix is trivial: add the respective file, plugins/RxJavaPlugins.html to the gradle code:

https://github.com/ReactiveX/RxJava/blob/2.x/gradle/javadoc_cleanup.gradle#L13

You can verify the outcome by running ./gradlew clean javadocCleanup and checking the respective file in your browser.

(Note that probably there is no way to fix the long signature displays, only those unnecessary newlines).