README installation instructions are incorrect

[mark-ross]

Hello!

I encountered an issue following the README instructions when adding this to the project.

The README instruction say to use

go get github.com/splitio/go-client/

However, this produces an error:

go: github.com/splitio/go-client: no matching versions for query "upgrade"

I believe the correct installation command is:

go get github.com/splitio/go-client/v6/splitio@latest

And as an FYI, I'm running Go v1.25.1.

While not a big issue, it might save folks some time who are looking to integrate this in their codebases.

Thanks for your time!