Update group_cities.py

kyu_6/rotate_the_letters_of_each_element/group_cities.py:10 in public function `group_cities`:
        D401: First line should be in imperative mood (perhaps 'Group', not 'Grouping')

        [Docstring] prescribes the function or method's effect as a command:
        ("Do this", "Return that"), not as a description; e.g. don't write
        "Returns the pathname ...".

kyu_6/rotate_the_letters_of_each_element/group_cities.py

@@ -8,7 +8,7 @@
 
 def group_cities(seq: list) -> list:
     """
-    Grouping cities.
+    Group cities.
 
     A function that given a sequence of strings, groups the elements
     that can be obtained by rotating others, ignoring upper or lower cases.