--help: clarify default values

[vitaly-zdanevich]

This is what we have now:

$ docker run ghcr.io/openzim/zimit zimit --help

  --scopeType {page,page-spa,prefix,host,domain,any,custom}
                        A predfined scope of the crawl. For more
                        customization, use 'custom' and set scopeIncludeRx
                        regexes

[benoit74]

Thank you, I think specifying default values should be done on all settings, not just this one. PR welcomed ^^