Fix Deprecation with Symfony Yaml Component v3.2+

[elkangaroo]

Fix syntax in debug.yml, that leads to a deprecation warning with Symfony 3.2+

Using a colon that is not followed by an indication character (i.e. " ", ",", "[", "]", "{", "}" is deprecated since version 3.2 and will throw a ParseException in 4.0: 1x

[crashbtz]

+1