Fix YAML parsing error in `url_preview_accept_language` (#12785)

changelog.d/12785.doc

+Fix invalid YAML syntax in the example documentation for the `url_preview_accept_language` config option.

docs/usage/configuration/config_documentation.md

@@ -1194,7 +1194,7 @@ For more information on using Synapse with Postgres,
 see [here](../../postgres.md).
 
 Example SQLite configuration:
-```
+```yaml
 database:
   name: sqlite3
   args:
@@ -1202,7 +1202,7 @@ database:
 ```
 
 Example Postgres configuration:
-```
+```yaml
 database:
   name: psycopg2
   txn_limit: 10000
@@ -1679,10 +1679,10 @@ Defaults to "en".
 Example configuration:
 ```yaml
  url_preview_accept_language:
-   - en-UK
-   - en-US;q=0.9
-   - fr;q=0.8
-   - *;q=0.7
+   - 'en-UK'
+   - 'en-US;q=0.9'
+   - 'fr;q=0.8'
+   - '*;q=0.7'
 ```
 ----
 Config option: `oembed`