docs(Widget): remove outdated return header from set_loading method docstring

This method was updated to use a different mechanism since its original
implementation and the docstring remained unchanged.

Author: ddkasa

src/textual/widget.py

@@ -957,9 +957,6 @@ def set_loading(self, loading: bool) -> None:
 
         Args:
             loading: `True` to put the widget into a loading state, or `False` to reset the loading state.
-
-        Returns:
-            An optional awaitable.
         """
         LOADING_INDICATOR_CLASS = "-textual-loading-indicator"
         if loading: