Fix missing completion for AppType 'cnb'

[marckhouzam]

Description of the Change

Missing completion choice cnb for the AppType when creating an app.

Why Is This PR Valuable?

Help the user know the possible choices for an AppType

Applicable Issues

NA

How Urgent Is The Change?

Not urgent

Manual testing

$ cat ~/cf.zsh
# zsh completion for Cloud Foundry CLI

_cf-cli() {
    # All arguments except the first one
    args=("${COMP_WORDS[@]:1:$COMP_CWORD}")
    # Only split on newlines
    local IFS=$'\n'
    # Call completion (note that the first element of COMP_WORDS is
    # the executable itself)
    COMPREPLY=($(GO_FLAGS_COMPLETION=1 ${COMP_WORDS[0]} "${args[@]}"))
    return 0
}
autoload -U +X bashcompinit && bashcompinit
complete -F _cf-cli cf
$ source ~/cf.zsh

# Notice that `cnb` is now shown 
$ out/cf create-app myapp --app-type <TAB>
buildpack  cnb        docker