Update README install instructions

[wcampbell0x2a]

As of 1.23.3 (or earlier), go get is no longer supported outside a module.
Error:

$ go get github.com/plexsystems/pacmod
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'.

Working:

$ go install github.com/plexsystems/pacmod@v0.6.0