Installation instructions to run go get don't work

[amardeep]

Bug 🐞

Running go get -u github.com/nikitavoloboev/gitupdate fails with following error:

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'.

Running following does work:

go install  github.com/nikitavoloboev/gitupdate@latest

➜  ~ go version
go version go1.18.3 darwin/arm64

[nikitavoloboev]

Thanks. Fixed the readme. ♥️