Fix small grammar mistake.

Author: birkenfeld

Doc/glossary.rst

@@ -759,7 +759,7 @@ Glossary
 
    statement
       A statement is part of a suite (a "block" of code).  A statement is either
-      an :term:`expression` or a one of several constructs with a keyword, such
+      an :term:`expression` or one of several constructs with a keyword, such
       as :keyword:`if`, :keyword:`while` or :keyword:`for`.
 
    struct sequence