Fix lexer_callback arg in stub for Lark constructor

lark-stubs/lark.pyi

@@ -56,7 +56,7 @@ class Lark:
         keep_all_tokens: bool = False,
         propagate_positions: bool = False,
         maybe_placeholders: bool = False,
-        lexer_callbacks: Dict[str, Callable[[Token], Token]]
+        lexer_callbacks: Optional[Dict[str, Callable[[Token], Token]]] = None
     ):
         ...