Fix the recently added example in the man page.
Fix the recently added example in the man page.

- Improve the language
- Be consistent with the previous example about a trailing slash
- Don't unnecessarily quote :, it is not a shell metacharacter.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo