3.x: Fix marbles of first(T) (#7400)

Author: akarnokd

src/main/java/io/reactivex/rxjava3/core/Flowable.java

@@ -10148,7 +10148,7 @@ public final Maybe<T> firstElement() {
      * Returns a {@link Single} that emits only the very first item emitted by this {@code Flowable}, or a default
      * item if this {@code Flowable} completes without emitting anything.
      * <p>
-     * <img width="640" height="285" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/first.s.png" alt="">
+     * <img width="640" height="298" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Flowable.first.s.png" alt="">
      * <dl>
      *  <dt><b>Backpressure:</b></dt>
      *  <dd>The operator honors backpressure from downstream and consumes the current {@code Flowable} in a bounded manner.</dd>

src/main/java/io/reactivex/rxjava3/core/Observable.java

@@ -8953,7 +8953,7 @@ public final Maybe<T> firstElement() {
      * Returns a {@link Single} that emits only the very first item emitted by the current {@code Observable}, or a default item
      * if the current {@code Observable} completes without emitting any items.
      * <p>
-     * <img width="640" height="285" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/first.s.png" alt="">
+     * <img width="640" height="283" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/first.s.png" alt="">
      * <dl>
      *  <dt><b>Scheduler:</b></dt>
      *  <dd>{@code first} does not operate by default on a particular {@link Scheduler}.</dd>