Correct example in runtime_troubles.rst (#12356)

`SupportsLessThan` was removed from `_typeshed` in 
python/typeshed#6583 and replaced with 
a new `SupportsRichComparison` type, which better reflects the 
types at runtime.

Fixes #12355

Author: AlexWaygood

docs/source/runtime_troubles.rst

@@ -254,7 +254,7 @@ sections, these can be dealt with by using :ref:`typing.TYPE_CHECKING
 
    from typing import TYPE_CHECKING
    if TYPE_CHECKING:
-       from _typeshed import SupportsLessThan
+       from _typeshed import SupportsRichComparison
 
 .. _generic-builtins: