Fix installation command in Development Setup

[theycallhermax]

When running go get -d github.com/writefreely/writefreely/cmd/writefreely, go outputs this:

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'

In this PR, I have fixed the installation step in the "Development Setup" article, and the changing of the directory part too.