Add marble diagram to the Single.never method (#6074)

* Add marble diagram to the Single.never method

* Use correct marble diagram URL for Single.never method

src/main/java/io/reactivex/Single.java

@@ -1162,6 +1162,8 @@ public static <T> Flowable<T> mergeDelayError(
 
     /**
      * Returns a singleton instance of a never-signalling Single (only calls onSubscribe).
+     * <p>
+     * <img width="640" height="244" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.never.png" alt="">
      * <dl>
      * <dt><b>Scheduler:</b></dt>
      * <dd>{@code never} does not operate by default on a particular {@link Scheduler}.</dd>
@@ -2603,7 +2605,7 @@ public final T blockingGet() {
      * Example:
      * <pre><code>
      * // Step 1: Create the consumer type that will be returned by the SingleOperator.apply():
-     * 
+     *
      * public final class CustomSingleObserver&lt;T&gt; implements SingleObserver&lt;T&gt;, Disposable {
      *
      *     // The downstream's SingleObserver that will receive the onXXX events