[docs] Another bug in Python 3.7 docs.

[Никита Люшненко]

https://docs.python.org/3/reference/expressions.html#yield-expressions

Term "sub-generator" is used in PEP 280 without dash ("-") symbol - "subgenerator". At the same time it is used as "sub-generator" 2 times on this page:

"generator (by returning a value from the sub-generator)." - here.

"The proposal to introduce the yield_from syntax, making delegation to sub-generators easy." - and here.

-- 
Sincerely,
ZooDSS
ZooDSS at yandex.ru