Update src/textual/pilot.py

Co-authored-by: Darren Burns <[email protected]>

Author: willmcgugan

src/textual/pilot.py

@@ -110,7 +110,7 @@ async def mouse_down(
         the offset specified and it must be within the visible area of the screen.
 
         Args:
-            selector: A widget or selector used as an origin
+            widget: A widget or selector used as an origin
                 for the event offset. If this is not specified, the offset is interpreted
                 relative to the screen. You can use this parameter to try to target a
                 specific widget. However, if the widget is currently hidden or obscured by