Update README.md fix install method

[zrthstr]

Update install method as go get -u is outdated and not working anymore.

go get -u repuURL will no longer work. See error message below.

go install repoURL@version works..

Old error:

% go get -u github.com/visma-prodsec/confused
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'.