add missing translation for italian translation (#4168)

add missing translation for italian translation

Resources/translations/SonataAdminBundle.it.xliff

@@ -464,7 +464,7 @@
             </trans-unit>
             <trans-unit id="label_unknown_user">
                 <source>label_unknown_user</source>
-                <target>label_unknown_user</target>
+                <target>sconosciuto</target>
             </trans-unit>
         </body>
     </file>