#15 Fix usage description

[Anonymous]

The syntax of syncmail when using syncmail --help is
incorrect.

If we use ./syncmail [options] <%S> email-addr
[email-addr..], the collect_diff will fail because it
expects the previous and the new version of the file.
Correct usage should read: ./syncmail [options] %{sVv}
email-addr [email-addr..].

[Allen R. Marshall]

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user_id=147690

And, don't allow %{sVvt} until support for t expansion (tags)
is implemented.