Fix parameter names in assertIn() docs (GH-18829)
The names "member" and "container" for the arguments are also used in the module and shown with the help() function, and are immediately understandable in this context, contrary to "first" and "second".
diff --git a/Doc/library/unittest.rst b/Doc/library/unittest.rst
index e2e4f2c..b2e16cf 100644
--- a/Doc/library/unittest.rst
+++ b/Doc/library/unittest.rst
@@ -910,10 +910,10 @@
.. versionadded:: 3.1
- .. method:: assertIn(first, second, msg=None)
- assertNotIn(first, second, msg=None)
+ .. method:: assertIn(member, container, msg=None)
+ assertNotIn(member, container, msg=None)
- Test that *first* is (or is not) in *second*.
+ Test that *member* is (or is not) in *container*.
.. versionadded:: 3.1