selftest: net: fix typo in altname test

If altname deletion of the short alternative name fails, the error
message printed is: "Failed to add short alternative name".
This is obviously a typo, as we are testing altname deletion.

Fix this using a proper error message.

Fixes: f95e6c9c4617 ("selftest: net: add alternative names test")
Signed-off-by: Andrea Claudi <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/altnames.sh

···
 	check_err $? "Got unexpected long alternative name from link show JSON"
 
 	ip link property del $DUMMY_DEV altname $SHORT_NAME
-	check_err $? "Failed to add short alternative name"
+	check_err $? "Failed to delete short alternative name"
 
 	ip -j -p link show $SHORT_NAME &>/dev/null
 	check_fail $? "Unexpected success while trying to do link show with deleted short alternative name"