Fix app.keys type

Author: pwizla

docusaurus/docs/dev-docs/configurations/server.md

@@ -23,7 +23,7 @@ The `./config/server.js` file can include the following parameters:
 |------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------|---------------------|
 | `host`<br/><br/>❗️ _Mandatory_     | Host name                                                                                                                                                                                                                                                                                                                                                                   | string            | `localhost`         |
 | `port`<br/><br/>❗️ _Mandatory_     | Port on which the server should be running.                                                                                                                                                                                                                                                                                                                                 | integer           | `1337`              |
-| `app.keys`<br/><br/>❗️ _Mandatory_ | Declare session keys (based on [Koa session](https://github.com/koajs/session/blob/master/Readme.md)), which is used by the `session` middleware for the Users & Permissions plugin and the Documentation plugin.                                                                                                                                                           | string            | `undefined`         |
+| `app.keys`<br/><br/>❗️ _Mandatory_ | Declare session keys (based on [Koa session](https://github.com/koajs/session/blob/master/Readme.md)), which is used by the `session` middleware for the Users & Permissions plugin and the Documentation plugin.                                                                                                                                                           | array of strings            | `undefined`         |
 | `socket`                           | Listens on a socket. Host and port are cosmetic when this option is provided and likewise use `url` to generate proper urls when using this option. This option is useful for running a server without exposing a port and using proxy servers on the same machine (e.g [Heroku nginx buildpack](https://github.com/heroku/heroku-buildpack-nginx#requirements-proxy-mode)) | string \| integer | `/tmp/nginx.socket` |
 | `emitErrors`                       | Enable errors to be emitted to `koa` when they happen in order to attach custom logic or use error reporting services.                                                                                                                                                                                                                                                      | boolean           | `false`             |
 | `url`                              | Public url of the server. Required for many different features (ex: reset password, third login providers etc.). Also enables proxy support such as Apache or Nginx, example: `https://mywebsite.com/api`. The url can be relative, if so, it is used with `http://${host}:${port}` as the base url. An absolute url is however recommended.                                | string            | `''`                |