quickstart: add ./ to run VPP with relative path

VPP version 22.02 has a known issue and doesn't support invoke with
relative path without './'.
Add './' to invoke VPP with relative path in the quickstart guide.

Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>

Change-Id: Ibcc96bd0e7df7ced96367e10067bd40abb2b5db5