yama: clarify ptrace_scope=2 in Yama documentation

Current phrasing is ambiguous since it's unclear if attaching to a
children through PTRACE_TRACEME requires CAP_SYS_PTRACE. Rephrase the
sentence to make that clear.

Signed-off-by: Yves-Alexis Perez <corsac@corsac.net>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>