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 <alok.a.tiwari@oracle.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250921192111.1567498-1-alok.a.tiwari@oracle.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>