[AIRFLOW-3127] Fix out-dated doc for Celery SSL (apache#3967)

Now in `airflow.cfg`, for Celery-SSL, the item names are
"ssl_active", "ssl_key", "ssl_cert", and "ssl_cacert".
(since PR https://github.com/apache/incubator-airflow/pull/2806/files)

But in the documentation
https://airflow.incubator.apache.org/security.html?highlight=celery
or
https://github.com/apache/incubator-airflow/blob/master/docs/security.rst,
it's "CELERY_SSL_ACTIVE", "CELERY_SSL_KEY", "CELERY_SSL_CERT", and
"CELERY_SSL_CACERT", which is out-dated and may confuse readers.

Author: XD-DENG

docs/security.rst

@@ -342,10 +342,10 @@ certs and keys.
 .. code-block:: bash
 
     [celery]
-    CELERY_SSL_ACTIVE = True
-    CELERY_SSL_KEY = <path to key>
-    CELERY_SSL_CERT = <path to cert>
-    CELERY_SSL_CACERT = <path to cacert>
+    ssl_active = True
+    ssl_key = <path to key>
+    ssl_cert = <path to cert>
+    ssl_cacert = <path to cacert>
 
 Impersonation
 -------------