Added bad-classmethod-argument example (#5996)

Author: matusvalo

doc/data/messages/b/bad-classmethod-argument/bad.py

@@ -0,0 +1,5 @@
+class Klass:
+
+    @classmethod
+    def get_instance(self):  # [bad-classmethod-argument]
+        return self()

doc/data/messages/b/bad-classmethod-argument/good.py

@@ -0,0 +1,5 @@
+class Klass:
+
+    @classmethod
+    def get_instance(cls):
+        return cls()