documentation: fix @url syntax in developer.texi

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

doc/developer.texi

@@ -324,7 +324,7 @@ send a reminder by email. Your patch should eventually be dealt with.
     Was the patch generated with git format-patch or send-email?
 @item
     Did you sign off your patch? (git commit -s)
-    See url{http://kerneltrap.org/files/Jeremy/DCO.txt} for the meaning
+    See @url{http://kerneltrap.org/files/Jeremy/DCO.txt} for the meaning
     of sign off.
 @item
     Is the patch against latest FFmpeg git master branch?