man/systemd.xml: change the signal names to kill systemd

Using the signal name to put systemd in debug mode with bash results in:
 $ kill -s SIGRTMIN+22 1
 bash: kill: SIGRTMIN+22: invalid signal specification

whereas this works:
 $ kill -s SIGRTMAX-8 1

/usr/bin/kill understands both signal names, so just change them to the
bash names.