[FrameworkBundle] fix YAML syntax

Author: xabbuh

Tests/DependencyInjection/Fixtures/yml/assets.yml

@@ -1,7 +1,7 @@
 framework:
     assets:
         version: SomeVersionScheme
-        version_format: %%s?version=%%s
+        version_format: '%%s?version=%%s'
         base_urls: http://cdn.example.com
         packages:
             images_path:
@@ -11,7 +11,7 @@ framework:
                 base_urls: ["http://images1.example.com", "http://images2.example.com"]
             foo:
                 version: 1.0.0
-                version_format: %%s-%%s
+                version_format: '%%s-%%s'
             bar:
                 base_urls: ["https://bar2.example.com"]
             bar_null_version: