[docs] Several typos in Python 3.7 docs.

[Никита Люшненко]

https://docs.python.org/3/reference/expressions.html#asynchronous-generator-functions
"function as a asynchronous generator function." - it's better to say "function as an asynchronous generator function."

https://docs.python.org/3/reference/expressions.html#agen.__anext__
"When an asynchronous generator function is resumed with a __anext__() method" - may be better "...function is resumed with an __anext__() method".

"the awaitable instead raises an StopAsyncIteration exception" - at the same time here may be better to use "the awaitable instead raises a StopAsyncIteration exception".

https://docs.python.org/3/reference/expressions.html#agen.athrow
"an StopAsyncIteration exception is raised" - here is the same situation as previously. "a" instead of "an" may be better.

Here goes very special case, and if i am wrong you can laugh on me as much as you wish! =)

https://docs.python.org/3/reference/expressions.html#parenthesized-forms
"Since tuples are immutable, the rules for literals apply" - Tuples are immutable, but not literals. So may be "the same rules as for literals apply" is better? Or does it apply exactly those rules, that apply for literals? "()" - this is a literal, that yelds an empty tuple. But this doesn't connect, or i am wrong? If i am - then i am very sorry.

Again, if i mistook something - i am sorry. Thanks for the good work. This is the best documentation i've ever seen.

-- 
Sincerely,
ZooDSS
ZooDSS at yandex.ru