selftests: rtnetlink: correct error message in rtnetlink.sh fou test

The rtnetlink FOU selftest prints an incorrect string:
"FAIL: fou"s. Change it to the intended "FAIL: fou" by
removing a stray character in the end_test string of the test.

Signed-off-by: Alok Tiwari <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/rtnetlink.sh

···
 	run_cmd_fail ip -netns "$testns" fou del port 9999
 	run_cmd ip -netns "$testns" fou del port 7777
 	if [ $ret -ne 0 ]; then
-		end_test "FAIL: fou"s
+		end_test "FAIL: fou"
 		return 1
 	fi