Fix cookie auth parameter in docs (#556)

Cookie authentication uses the "cookies" parameter, not "cookie".

See atlassian/rest_client.py

Author: modelrockettier

docs/index.rst

@@ -133,19 +133,19 @@ Or reuse cookie file:
 
     jira = Jira(
         url='http://localhost:8080',
-        cookie=cookie_dict)
+        cookies=cookie_dict)
 
     confluence = Confluence(
         url='http://localhost:8090',
-        cookie=cookie_dict)
+        cookies=cookie_dict)
 
     bitbucket = Bitbucket(
         url='http://localhost:7990',
-        cookie=cookie_dict)
+        cookies=cookie_dict)
 
     service_desk = ServiceDesk(
         url='http://localhost:8080',
-        cookie=cookie_dict)
+        cookies=cookie_dict)
 
 To authenticate to the Atlassian Cloud APIs: