[FrameworkBundle] Check for 'xlf' instead of 'xliff'

There is no translation writer format named 'xliff', but 'xlf' only. So the TranslationUpdateCommand can't be called with 'output-format' == 'xliff' and the version info will never be shown.

src/Symfony/Bundle/FrameworkBundle/Command/TranslationUpdateCommand.php

@@ -125,7 +125,7 @@ EOF
                 }
             }
 
-            if ($input->getOption('output-format') == 'xliff') {
+            if ($input->getOption('output-format') == 'xlf') {
                 $output->writeln('Xliff output version is <info>1.2</info>');
             }
         }