fix: change attribute name from markdown to md in the Plugin class

Author: impocode

src/mistune/plugins/__init__.py

@@ -24,7 +24,7 @@
 
 
 class Plugin(Protocol):
-    def __call__(self, markdown: "Markdown") -> None: ...
+    def __call__(self, md: "Markdown") -> None: ...
 
 _cached_modules: Dict[str, Plugin] = {}