Docs: fix TypeError in code example (#1413)

Fix TypeError of 'class MyProto(Protocol)'

TypeError: Too few arguments for typing.Mapping; actual 1, expected 2

Author: ozgurturkiye

docs/source/best_practices.rst

@@ -86,7 +86,7 @@ Maybe::
 
     class MyProto(Protocol):
         def foo(self) -> list[int]: ...
-        def bar(self) -> Mapping[str]: ...
+        def bar(self) -> Mapping[str, str]: ...
 
 Avoid union return types, since they require ``isinstance()`` checks.
 Use ``Any`` or ``X | Any`` if necessary.