improve `roller keys export` usage output

[mtsitrin]

Currently

./roller keys export                                                                                                                      ✔
Error: accepts 1 arg(s), received 0
Usage:
  roller keys export <key-id> [flags]

Flags:
  -h, --help   help for export

Global Flags:
      --home string   The directory of the roller config files (default "/Users/mtsitrin/.roller")

This usage output should be improved with documentation of the possible values for <key-id>