fix description for ptrace in seccomp.md
fix description for ptrace in seccomp.md

The existing description leads the reader to believe that dropping CAP_SYS_PTRACE already blocks all ptraces. That is not true, it only blocks ptracing arbitrary processes. Ptracing child processes is still allowed.
thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io