[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
@@ -6,9 +8,9 @@
 # to you under the Apache License, Version 2.0 (the
 # "License"); you may not use this file except in compliance
 # with the License.  You may obtain a copy of the License at
-# 
+#
 #   http://www.apache.org/licenses/LICENSE-2.0
-# 
+#
 # Unless required by applicable law or agreed to in writing,
 # software distributed under the License is distributed on an
 # "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY