[AIRFLOW-3643] Add shebang to docs/start_doc_server.sh (#4650)

Since this script uses bash syntax, shebang needs to be added.

docs/start_doc_server.sh

@@ -1,3 +1,5 @@
+#!/usr/bin/env bash
+
 #
 # Licensed to the Apache Software Foundation (ASF) under one
 # or more contributor license agreements.  See the NOTICE file