fix: yaml snippet for test_command (#859)

fix: yaml snippet for `test_command`

Quick documentation fix since the header only is not clear that default_identifier have to be under a test_command object

Reviewed-by: Laura Barcziová

docs/configuration/index.md

@@ -606,7 +606,8 @@ For instance, rather than specifying frequently used job via `--identifier` each
 you can configure the following:
 
 ```yaml
-default_identifier: my-id
+test_command:
+  default_identifier: my-id
 ```
 As a result, invoking `/packit test` will be automatically interpreted as `/packit test --identfier my-id`.
 
@@ -618,9 +619,10 @@ This allows commonly used job combinations to be triggered without the need for
 For instance, rather than specifying frequently used jobs via `--labels` each time, such as `/packit test --labels regression,upgrade`, 
 you can configure the following:
 ```yaml
-default_labels:
-  - regression
-  - upgrade
+test_command:
+  default_labels:
+    - regression
+    - upgrade
 ```
 As a result, invoking `/packit test` will be automatically interpreted as `/packit test --labels regression,upgrade`.