Change mtail service to read /dev/stdin instead of "-"

The shell alias "-" should work for stdin, but on one of my servers it
was not working and stracing the process showed it kept trying to open a
file named /-. I do not know how or why this was the case, but altering
it to read /dev/stdin instead solved the issue.

cmdeploy/src/cmdeploy/mtail/mtail.service.j2

@@ -3,7 +3,7 @@ Description=mtail
 
 [Service]
 Type=simple
-ExecStart=/bin/sh -c "journalctl -f -o short-iso -n 0 | /usr/local/bin/mtail --address={{ address }} --port={{ port }} --progs /etc/mtail --logtostderr --logs -"
+ExecStart=/bin/sh -c "journalctl -f -o short-iso -n 0 | /usr/local/bin/mtail --address={{ address }} --port={{ port }} --progs /etc/mtail --logtostderr --logs /dev/stdin"
 Restart=on-failure
 
 [Install]