samples/custom-bootstrap/README.md: use relative file path (#28559)
samples/custom-bootstrap/README.md: use relative file path (#28559)

Signed-off-by: Xiang Dai <c54c64cf7043291b886753756d851be401eacd91@foxmail.com>
istio/istio,istio/istio,istio/istio,istio/istio,istio/istio