docs: drop the emphasis from the app.ini heading

markdown-link-check derives the anchor from the raw heading text, where the underscores of the emphasis survive
slugification as `additional-_appini_-settings`. Gitea and GitHub slugify the rendered heading instead, so the
anchor really is `additional-appini-settings` and the table of contents entry was correct all along - the checker
simply could not see it.

The entry appeared with the last regeneration of the README, which is why the job started failing now even though
the heading has been around for much longer. Removing the emphasis makes both slugifiers agree without changing the
anchor, so existing deep links keep working.

Co-authored-by: Copilot <copilot@github.com>

README.md

@@ -19,7 +19,7 @@
     - [Gateway API](#gateway-api)
     - [Session, Cache and Queue](#session-cache-and-queue)
   - [Single-Pod Configurations](#single-pod-configurations)
-  - [Additional _app.ini_ settings](#additional-appini-settings)
+  - [Additional app.ini settings](#additional-appini-settings)
     - [User defined environment variables in app.ini](#user-defined-environment-variables-in-appini)
   - [External Database](#external-database)
   - [Ports and external url](#ports-and-external-url)
@@ -397,7 +397,7 @@ If HA is not needed/desired, the following configurations can be used to deploy
 
    </details>
 
-### Additional _app.ini_ settings
+### Additional app.ini settings
 
 > **The [generic](https://docs.gitea.com/administration/config-cheat-sheet#overall-default)
 > section cannot be defined that way.**