README: Install path is deprecated with latest Go

[zeisss]

I am getting this (in my home directory, with go version go1.17.2 darwin/amd64):

% go get github.com/orijtech/structslop/cmd/structslop
go: downloading github.com/orijtech/structslop v0.0.6
go: downloading golang.org/x/tools v0.0.0-20200917221617-d56e4e40bc9d
go: downloading github.com/dave/dst v0.26.2
go: downloading golang.org/x/mod v0.3.0
go get: installing executables with 'go get' in module mode is deprecated.
	Use 'go install pkg@version' instead.
	For more information, see https://golang.org/doc/go-get-install-deprecation
	or run 'go help get' or 'go help install'.

With Go 1.17 the new way is:

% go install github.com/orijtech/structslop/cmd/structslop@latest