Fix a typo in the JavaDoc

Author: akarnokd

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

@@ -14017,7 +14017,7 @@ public final Completable switchMapCompletable(@NonNull Function<? super T, ? ext
      *  <dt><b>Error handling:</b></dt>
      *  <dd>Errors of this {@code Flowable} and all the {@code CompletableSource}s, who had the chance
      *  to run to their completion, are delayed until
-     *  all of the terminate in some fashion. At this point, if there was only one failure, the respective
+     *  all of them terminate in some fashion. At this point, if there was only one failure, the respective
      *  {@code Throwable} is emitted to the dowstream. It there were more than one failures, the
      *  operator combines all {@code Throwable}s into a {@link io.reactivex.exceptions.CompositeException CompositeException}
      *  and signals that to the downstream.