bpf/docs: Update README for most recent vmtest.sh

Since commit 40b09653 ("selftests/bpf: Adjust vmtest.sh to use local
kernel configuration") the vmtest.sh script no longer downloads a kernel
configuration but uses the local, in-repository one.
This change updates the README, which still mentions the old behavior.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221017232458.1272762-1-deso@posteo.net