Update flow go SDK installation instructions

[alilloig]

Problem

In the flow go sdk docs we are still using go get instead of go install. It needs to be updated.

[devbugging]

Good find. Thank you.

[alilloig]

All merit to our docs feedback form, I'm just monitorizing it https://docs.google.com/forms/d/1oQ_Fy7K4pRmC83cap-AwRGO3bRbSDZc0DuL5kfJ3JJo