Change outdated command in INSTRUCTIONS.md

[mbledkowski]

Hello,

Since Go 1.16 go get for global installs is no longer supported:

$ go get -u github.com/dgraph-io/ratel                                                                                                                                                                                                                                                                                                                           
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'.

Nowadays, you should use go install for global installs.

Best regards, Maciej Błędkowski

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