Fix some ReST indentation issues in docstrings

src/_pytest/mark/structures.py

@@ -193,6 +193,7 @@ class MarkDecorator:
             pass
 
     When a MarkDecorator instance is called it does the following:
+
     1. If called with a single class as its only positional argument and no
        additional keyword arguments, it attaches itself to the class so it
        gets applied automatically to all test cases found in that class.