[pre-commit.ci] auto fixes from pre-commit.com hooks

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Author: pre-commit-ci[bot]

docs/source/standalone.md

@@ -184,7 +184,7 @@ JupyterHub uses a lot of environment variables to specify how the launched app s
 This list is a small overview of all used variables and what they contain and are used for.
 
 | Variable                                                                        | Explanation                                                                                                                                                                            | Typical Value                              |
-|---------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------|
+| ------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------ |
 | `JUPYTERHUB_SERVICE_URL`                                                        | URL where the server should be listening. Used to find the Address and Port to start the server on.                                                                                    | `http://127.0.0.1:5555`                    |
 | `JUPYTERHUB_SERVICE_PREFIX`                                                     | An URL Prefix where the root of the launched application should be hosted. E.g., when set to `/user/name/`, then the root of the proxied application should be `/user/name/index.html` | `/services/service-name/` or `/user/name/` |
 | `JUPYTERHUB_ACTIVITY_URL`                                                       | URL where to send activity notifications to.                                                                                                                                           | `$JUPYTERHUB_API_URL/user/name/activity`   |