docs: update countries_client_provider documentation comment

- Clarify the purpose of the countriesClientProvider middleware
- Improve code readability by adding a blank line between the comment and code

Author: fulleni

lib/src/providers/countries_client_provider.dart

@@ -4,7 +4,7 @@ import 'package:ht_countries_inmemory/ht_countries_inmemory.dart';
 
 /// Provides an instance of [HtCountriesClient] to the request context.
 ///
-/// This middleware uses the inmemory implementation 
+/// This middleware uses the inmemory implementation
 /// [HtCountriesInMemoryClient].
 Middleware countriesClientProvider() {
   // Create the client instance once when the middleware is initialized.