Update info on reloading sysctl with systemd

[Jerry Quinn]

/etc/sysctl.d/README says that after making changes, you should run "service procps start" to activate the changes.

Since systemd is now the default init system, please update the README files to also say you can activate changes with:

systemctl restart systemd-sysctl

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: procps 2:3.3.16-1ubuntu2
ProcVersionSignature: Ubuntu 5.4.0-60.67-generic 5.4.78
Uname: Linux 5.4.0-60-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.14
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Tue Jan 12 09:23:57 2021
InstallationDate: Installed on 2020-05-29 (227 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: procps
UpgradeStatus: Upgraded to focal on 2020-11-04 (68 days ago)
modified.conffile..etc.sysctl.d.10-link-restrictions.conf: [deleted]