Fix git clone link (#260)
Fix git clone link (#260)

'$ git clone git://github.com/martin-helmich/prometheus-nginxlog-exporter'
no longer works as:
```
The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
```
Change to https git clone link
martin-helmich/prometheus-nginxlog-exporter,martin-helmich/prometheus-nginxlog-exporter,martin-helmich/prometheus-nginxlog-exporter