docs(option_list): fix typo in highlighted_option docstring

Author: TomJGooding

src/textual/widgets/_option_list.py

@@ -313,7 +313,7 @@ def option_count(self) -> int:
 
     @property
     def highlighted_option(self) -> Option | None:
-        """The currently highlighted options, or `None` if no option is highlighted,
+        """The currently highlighted option, or `None` if no option is highlighted.
 
         Returns:
             An Option, or `None`.