Docs for marking a batched BG migration finished

What does this MR do and why?

This MR adds a section for force-marking a BG migration finished in case it's broken.

Related to: #393216 (closed)

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• I have evaluated the MR acceptance checklist for this MR.

doc/update/background_migrations.md

@@ -266,7 +266,7 @@ arguments until the status query returns no rows.
 
 1. Run a reconfigure:
 
-     ```plaintext
+   ```plaintext
    sudo gitlab-ctl reconfigure
    ```
 
@@ -336,7 +336,7 @@ When the background migration is determined to be "safe" to skip, the migration
 WARNING:
 Make sure you create a backup before proceeding.
 
-```shell
+```ruby
 # Start the rails console
 
 connection = ApplicationRecord.connection # or Ci::ApplicationRecord.connection, depending on which DB was the migration scheduled