Change installation instructions to use speedtest_rs instead of librespeed-rs

[jniles]

The installation documentation in the wiki still refers to librespeed-rs as the service, but this has been updated to speedtest_rs in the systemd service file.

Please update the installation instructions to refer to the correct service name of speedtest_rs.service.

[SudoDios]

Ηι @jniles
Yes, you're right,
the names should be the same.