fix help for -a to correctly include label:name=key

[maximilien]

Current help for logs command -a option is missing label in format. So changing:

➜  kapp git:(logs-a-label) ./kapp logs -h
Print app's Pod logs

Usage:
  kapp logs [flags]

Aliases:
  logs, l

Flags:
  -a, --app string         Set app name (or label selector) (format: name, key=val, !key)
...

to

➜  kapp git:(logs-a-label) ./kapp logs -h
Print app's Pod logs

Usage:
  kapp logs [flags]

Aliases:
  logs, l

Flags:
  -a, --app string         Set app name (or label selector) (format: name, label:key=val, !key)
...

which is the correct way of specifying a label selector.