Rename "generator expressions" to "generator comprehensions"

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-07-15.00:27:13.795>
labels = ['docs']
title = 'Rename "generator expressions" to "generator comprehensions"'
updated_at = <Date 2018-08-14.00:53:51.586>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2018-08-14.00:53:51.586>
actor = 'rhettinger'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-07-15.00:27:13.795>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 34117
keywords = []
message_count = 5.0
messages = ['321670', '321676', '321678', '323484', '323496']
nosy_count = 5.0
nosy_names = ['brett.cannon', 'rhettinger', 'docs@python', 'serhiy.storchaka', 'GeekyShacklebolt']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue34117'
versions = []