Deprecate Observable.apply

Author: zsxwing

language-adaptors/rxjava-scala/src/examples/scala/rx/lang/scala/examples/RxScalaDemo.scala

@@ -578,7 +578,7 @@ class RxScalaDemo extends JUnitSuite {
 
   @Test def elementAtExample(): Unit = {
     val o = List("red", "green", "blue").toObservable
-    println(o(2).toBlockingObservable.single)
+    println(o.elementAt(2).toBlockingObservable.single)
   }
 
   @Test def elementAtOrDefaultExample(): Unit = {

language-adaptors/rxjava-scala/src/main/scala/rx/lang/scala/Observable.scala

@@ -2350,6 +2350,7 @@ trait Observable[+T]
    * @throws IndexOutOfBoundsException
    *             if index is less than 0
    * @see `Observable.elementAt`
+   * @deprecated("Use `elementAt`", "0.18.0")
    */
   def apply(index: Int): Observable[T] = elementAt(index)