Improve the grammar in the glossary item for `list` (GH-7727)

" ... access to elements is O(1)."

(cherry picked from commit 7469ff50)
Co-authored-by: Andrés Delfino <adelfino@gmail.com>

Doc/glossary.rst

@@ -474,7 +474,7 @@ Glossary
    list
       A built-in Python :term:`sequence`.  Despite its name it is more akin
       to an array in other languages than to a linked list since access to
-      elements are O(1).
+      elements is O(1).
 
    list comprehension
       A compact way to process all or part of the elements in a sequence and