Wrong ExecStart path in service file. 

Hi,

I installed the Debian packet  `mqtt2prometheus_0.1.7_linux_amd64.deb` on a Debian v12.1 system.

The service file states:
```
ExecStart=/opt/mqtt2prometheus/mqtt2prometheus -config /etc/mqtt2prometheus/config.yaml $ARGS
```
but the binary is in `/usr/bin/mqtt2prometheus`.

Thanx,
Henning