Edit dependency for gradle (ReactiveX#7023)

Since gradle 3.0.x the `compile` keyword is deprecated. [You should](https://developer.android.com/studio/build/dependencies#dependency_configurations) use 'implementation' instead.

Author: timkoerner

docs/Getting-Started.md

@@ -27,7 +27,7 @@ libraryDependencies += "io.reactivex.rxjava3" % "rxjava" % "3.0.4"
 
 and for Gradle:
 ```groovy
-compile 'io.reactivex.rxjava3:rxjava:3.0.4'
+implementation 'io.reactivex.rxjava3:rxjava:3.0.4'
 ```
 
 If you need to download the jars instead of using a build system, create a Maven `pom` file like this with the desired version: