doc: Add go install command in installation

[indradhanush]

Fixes this warning:

$ go get -u github.com/go-jira/jira/cmd/jira
  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'.