[docs] [issue31811] async and await missing from keyword list in lexical analysis doc

[F. Lamar]

F. Lamar <lamar.frankie at gmail.com> added the comment:

I a new contributor. This seems like a simple fix. I'd be happy to work on it. I will submit ar PR on or before 10-22-17

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nosy: +F. Lamar

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31811>
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