zephyr: Fix README
zephyr: Fix README

Fix the path for running gdb.

BRANCH=None
BUG=None
TEST=Ran gdb

Signed-off-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@google.com>
Change-Id: I1e375e917db2fd2280fca6668dc1b31185519c53
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3269055
Commit-Queue: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>
Tested-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>
Reviewed-by: Abe Levkoy <2e1ad7039298f93262cf06bbb457275fb3e2ecb5@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec