Improve the grammar in the glossary item for `list`

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

(cherry picked from commit 7469ff5017ec315a81e35913f19a32f0dbdf712e)

Co-authored-by: Andrés Delfino <adelfino@gmail.com>
diff --git a/Doc/glossary.rst b/Doc/glossary.rst
index 988842a..0f658b8 100644
--- a/Doc/glossary.rst
+++ b/Doc/glossary.rst
@@ -642,7 +642,7 @@
    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