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

Doc/glossary.rst

@@ -642,7 +642,7 @@ 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