Fix typo in contextvars docs (#98823)

Author: cburroughs

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