Fix EnsureGithubDirectiveStartWithPrefix rule with named directive

[alamirault]

There is 16 false-positive on symfony 5.4 branch

For example

components/property_info.rst ✘
  267: Please only use "Symfony" base namespace with Github directive
   ->  :method:`PropertyInfoExtractor::getTypes() <Symfony\\Component\\PropertyInfo\\PropertyInfoExtractor::getTypes>`
  284: Please only use "Symfony" base namespace with Github directive
   ->  The :method:`Type::getBuiltinType() <Symfony\\Component\\PropertyInfo\\Type::getBuiltinType>`
  295: Please only use "Symfony" base namespace with Github directive
   ->  The :method:`Type::isNullable() <Symfony\\Component\\PropertyInfo\\Type::isNullable>`
  303: Please only use "Symfony" base namespace with Github directive
   ->  is ``object``, the :method:`Type::getClassName() <Symfony\\Component\\PropertyInfo\\Type::getClassName>`
  309: Please only use "Symfony" base namespace with Github directive
   ->  The :method:`Type::isCollection() <Symfony\\Component\\PropertyInfo\\Type::isCollection>`
  327: Please only use "Symfony" base namespace with Github directive
   ->  available), via the :method:`Type::getCollectionKeyTypes() <Symfony\\Component\\PropertyInfo\\Type::getCollectionKeyTypes>`
  328: Please only use "Symfony" base namespace with Github directive
   ->  and :method:`Type::getCollectionValueTypes() <Symfony\\Component\\PropertyInfo\\Type::getCollectionValueTypes>`