Fix incorrectly documented flag name in man page

[Avaq]

The manpage states that --exclude can be used to ignore a given path, while the actual binary only accepts --exclude-path and returns an error for --exclude.

[jvirkki]

Thank you for noticing!

Please note that man/dupd is an autogenerated file so the change should be made first to the source file man/dupd.1

[Avaq]

@jvirkki whoops; done.