Fix description of output redirection for access_log for gunicorn (#2…

…8672)

As of gunicorn 19.7.0, default for access_log is stdout not stderr
and our documentation has not been updated to reflect that. We are
already past that (min version of gunicorn is 20.1.0, so the
documentation of access-log flag of ours was wrong. Having the
access_log in stdout rather than stderr also allows to redirect
the access log to a separate log sink in deployments like K8S.

airflow/cli/cli_parser.py

@@ -663,7 +663,7 @@ def string_lower_type(val):
 ARG_ACCESS_LOGFILE = Arg(
     ("-A", "--access-logfile"),
     default=conf.get("webserver", "ACCESS_LOGFILE"),
-    help="The logfile to store the webserver access log. Use '-' to print to stderr",
+    help="The logfile to store the webserver access log. Use '-' to print to stdout",
 )
 ARG_ERROR_LOGFILE = Arg(
     ("-E", "--error-logfile"),