Fix cookie auth parameter in docs

[modelrockettier]

The docs currently say to use the "cookie" parameter for cookie authentication, but the code actually uses the "cookies" parameter (plural).

See

atlassian-python-api/atlassian/rest_client.py

Lines 23 to 25 in 23370dd

	def __init__(self, url, username=None, password=None, timeout=60, api_root='rest/api', api_version='latest',
	verify_ssl=True, session=None, oauth=None, cookies=None, advanced_mode=None, kerberos=None,
	cloud=False, proxies=None):

[gonchik]

Thank you! :)
That's great to see all polishes