fix doc build, use noindex on the mark reference

RonnyPfannschmidt · RonnyPfannschmidt · commit 4df8f2b15333 · 2018-04-06T08:13:28.000+02:00
diff --git a/doc/en/mark.rst b/doc/en/mark.rst
@@ -29,6 +29,7 @@ which also serve as documentation.
 .. currentmodule:: _pytest.mark.structures
 .. autoclass:: Mark
 	:members:
+	:noindex:
 
 
 .. `marker-iteration`
