doc: change title of validate_config use case
-# Use Case: Ensuring valid configuration
+# Use Case: Validating Configuration with Specification
## Summary
-- **Title:** Ensuring valid configuration
+- **Title:** Validating Configuration with Specification
- **Scope:** `libelektra-kdb`, `spec`
- **Level:** Developer Goal, User Goal
- **Actors:** Elektra, Application, User
