PEP 636 typo fix (#1849)

Fix minor typo:
"But **_ would be redundant, so it not allowed."
--> "But **_ would be redundant, so it is not allowed."

pep-0636.rst

@@ -578,7 +578,7 @@ Several other key features:
 - Mapping patterns: ``{"bandwidth": b, "latency": l}`` captures the
   ``"bandwidth"`` and ``"latency"`` values from a dict.  Unlike sequence
   patterns, extra keys are ignored.  A wildcard ``**rest`` is also
-  supported.  (But ``**_`` would be redundant, so it not allowed.)
+  supported.  (But ``**_`` would be redundant, so it is not allowed.)
 
 - Subpatterns may be captured using the ``as`` keyword::