Update reference to github.com/spf13/cobra/cobra

https://github.com/codeedu/imersao-fullstack-fullcycle/blob/main/codepix/Dockerfile references `go get github.com/spf13/cobra/cobra`

The CLI command package has moved to github.com/spf13/cobra-cli. See https://github.com/spf13/cobra/issues/1597 for details.

To install the CLI, switch to `go get github.com/spf13/cobra-cli`

To install the library, shorten to `go get github.com/spf13/cobra`