sshkeys: add example to ssh-key import help text

[beardicus]

This is responding to some ideas.digitalocean.com feedback from a confused user. Adds an example and some more description to doctl compute ssh-key import.

Summary

• Add an Examples: block to doctl compute ssh-key import --help. Previously the subcommand had no example, while most neighbouring commands (e.g. firewalls, kubernetes, volumes, monitoring, registry) and newer commands (e.g. spaces keys) do.
• Expand the long description to mention the <key-name> argument and the --public-key-file flag, matching the style used for the neighbouring create subcommand.

Help output

Before:

Use this command to add a new SSH key to your account, using a local public key file.

Note that importing a key to your account will not add it to any Droplets

Usage:
  doctl compute ssh-key import <key-name> [flags]
...

After:

Use this command to add a new SSH key to your account, using a local public key file.

Specify a `<key-name>` for the key, and set the `--public-key-file` flag to the path of a local public key file, such as `~/.ssh/id_ed25519.pub`.

Note that importing a key to your account will not add it to any Droplets.

Usage:
  doctl compute ssh-key import <key-name> [flags]

Aliases:
  import, i

Examples:
doctl compute ssh-key import example-key --public-key-file ~/.ssh/id_ed25519.pub
...

[Bala-Nallamilli]

LGTM