document bug: fatal: repository 'github.com/librespeed/speedtest-go' does not exist

bug in [README.md](https://github.com/librespeed/speedtest-go/blob/master/README.md) 
```
> git clone github.com/librespeed/speedtest-go
fatal: repository 'github.com/librespeed/speedtest-go' does not exist
```
should be `git clone https://github.com/librespeed/speedtest-go`