Fix typo in PEP 487

pep-0487.txt

@@ -142,7 +142,7 @@ Initializing descriptors could simply be done in the
 ``__init_subclass__`` hook. But this would mean that descriptors can
 only be used in classes that have the proper hook, the generic version
 like in the example would not work generally. One could also call
-``__set_name__`` from whithin the base implementation of
+``__set_name__`` from within the base implementation of
 ``object.__init_subclass__``. But given that it is a common mistake
 to forget to call ``super()``, it would happen too often that suddenly
 descriptors are not initialized.