Fix typo in contextvars docs (GH-98823)

(cherry picked from commit 72fa57a)

Co-authored-by: cburroughs <[email protected]>

Author: miss-islington

Doc/library/contextvars.rst

@@ -110,7 +110,7 @@ Context Variables
 
       A read-only property.  Set to the value the variable had before
       the :meth:`ContextVar.set` method call that created the token.
-      It points to :attr:`Token.MISSING` is the variable was not set
+      It points to :attr:`Token.MISSING` if the variable was not set
       before the call.
 
    .. attribute:: Token.MISSING