Fix incorrect path in ENTRYPOINT example

The ENTRYPOINT example uses "/usr/bin/ls" as path, but `ls` is located at `/bin/ls` on ubuntu.

As suggested by @tianon, a path is not required at all, since `ENTRYPOINT` will be able to resolve the executable via `$PATH`

Docker-DCO-1.1-Signed-off-by: Sebastiaan van Stijn github@gone.nl (github: thaJeztah)