Use singular, we are talking about the access (GH-7727)

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