Updated documentation
Updated documentation

As per the reference of [v3 commands](https://docs.cloudfoundry.org/devguide/v3-commands.html), it can be understood that the command `v3-app` does not exist.


The step: [`$ cf v3-app myapp`](https://github.com/cloudfoundry/docs-dev-guide/blob/edee1d63a423ef5f94f67359282163a42dc95212/multiple-processes.html.md.erb#L79)  will error to:
> 'v3-app' is not a registered command. See 'cf help -a'

As per the v3-app commands page, the step
`$ cf v3-app myapp` needs to be changed to `$ cf app myapp`
cloudfoundry/docs-dev-guide