Fix struct sequence glossary entry grammar (GH-9030)

... by removing a superfluous "either".

Reported by Никита Люшненко on docs@.

Doc/glossary.rst

@@ -1012,7 +1012,7 @@ Glossary
 
    struct sequence
       A tuple with named elements. Struct sequences expose an interface similar
-      to :term:`named tuple` in that elements can either be accessed either by
+      to :term:`named tuple` in that elements can be accessed either by
       index or as an attribute. However, they do not have any of the named tuple
       methods like :meth:`~collections.somenamedtuple._make` or
       :meth:`~collections.somenamedtuple._asdict`. Examples of struct sequences