[generate:theme] fix libraries.yml syntax (#4187)

There is an indexation error in the template, the generated file is invalid

Author: lalop

templates/theme/libraries.yml.twig

@@ -7,10 +7,10 @@
      #js/your_js.js : {}
 {% for library in libraries %}
 {{ library.library_name }}:
-  version: {{ library.library_version }}
+   version: {{ library.library_version }}
    css:
     theme:
       #js/your_js.js : {}
    js:
      #js/your_js.js : {}
-{% endfor %}
+{% endfor %}