transmission.service: fix ignored ExecReload setting

This is what currently happens (from the journal log):

  [/nix/store/HASH-unit/transmission.service:27] Executable path is not absolute, ignoring: kill -HUP $MAINPID

Fix it by using absolute path to kill.
Merge pull request #161 from bjornfor/transmission

transmission.service: fix ignored ExecReload setting