Revise Management console API Authentication curl example

Current `curl` example doesn't work if the Management Console password contains `@` character.

```
#!/bin/bash
set -eu

export your_amazing_password=foo@bar
export hostname=my-ghes.example.com

curl -L https://api_key:${your_amazing_password}@${hostname}:8443/setup/api
```

```
$ bash maintenance-demo.sh
curl: (6) Could not resolve host: bar@my-ghes.example.com
```

`curl -u (--user)` option solves the problem.

Author: kyanny

content/rest/reference/enterprise-admin.md

@@ -183,7 +183,7 @@ $ curl -L 'https://<em>hostname</em>:<em>admin_port</em>/setup/api?api_key=<em>y
 You can also use standard HTTP authentication to send this token. For example:
 
 ```shell
-$ curl -L 'https://api_key:<em>your-amazing-password</em>@<em>hostname</em>:<em>admin_port</em>/setup/api'
+$ curl -L -u "api_key:<em>your-amazing-password</em>" 'https://<em>hostname</em>:<em>admin_port</em>/setup/api'
 ```
 
 {% for operation in currentRestOperations %}