docs: 📝 update instructions for source installation

[minhtrancccp]

as i tried to check for the fix to the erroneous failed to read config file message (as shown in #488), i first executed go install github.com/noborus/ov@latest, which just fetched the latest release build and did not included the fix (i wasn't aware of how to install from the master branch at the time). when i followed the option of go get -d github.com/noborus/ov, i got an error from go itself, saying:

go: go.mod file not found in current directory or any parent directory.
        'go get' is no longer supported outside a module.
        To build and install a command, use 'go install' with a version,
        like 'go install example.com/cmd@latest'
        For more information, see https://golang.org/doc/go-get-install-deprecation
        or run 'go help get' or 'go help install'.

i assume that those without much knowledge of go like myself but want to have a local clone of the repo would be confused by this error message, which can be easily resolved by cloning the repo directly with git clone or gh repo clone instead, hence the reason why this commit was made.

[noborus]

Thanks for your pull request. Documentation improvements are welcome.

[noborus]

Thank you for the correction. I think it's good so I'll merge it. Please feel free to give your feedback anytime.