Change error message in ObservableFromArray

Changed error message from "The $i th element is null" to "The element at index $i is null".

Solves ReactiveX#6460

src/main/java/io/reactivex/internal/operators/observable/ObservableFromArray.java

@@ -102,7 +102,7 @@ void run() {
             for (int i = 0; i < n && !isDisposed(); i++) {
                 T value = a[i];
                 if (value == null) {
-                    downstream.onError(new NullPointerException("The " + i + "th element is null"));
+                    downstream.onError(new NullPointerException("The element at index " + i + " is null"));
                     return;
                 }
                 downstream.onNext(value);