Fix link in RenderObjectWidget doc comment (flutter#150600)

Fix a link in the `RenderObjectWidget` doc comment. The comment recommends subclassing `SingleChildRenderObjectElement` where it means to recommend subclassing `SingleChildRenderObjectWidget`.

Fixes flutter#150597.

Author: swrenn

packages/flutter/lib/src/widgets/framework.dart

@@ -1878,7 +1878,7 @@ abstract class InheritedWidget extends ProxyWidget {
 /// object widgets subclass one of:
 ///
 ///  * [LeafRenderObjectWidget], if the widget has no children.
-///  * [SingleChildRenderObjectElement], if the widget has exactly one child.
+///  * [SingleChildRenderObjectWidget], if the widget has exactly one child.
 ///  * [MultiChildRenderObjectWidget], if the widget takes a list of children.
 ///  * [SlottedMultiChildRenderObjectWidget], if the widget organizes its
 ///    children in different named slots.