Tweak README documentation, clarifying Redmine Custom Field types.

Hello. Thank you for the plugin. :)

Some small documentation tweaks, if I may. I think most important are the clarifications in the § Usage section; I misinterpreted the instructions for `owner-email` at first, and made a Project-type Custom Field instead of an Issue-type.

- change typos 'send' => 'sent'
- change typo 'repro' => 'repo'
- clarify Redmine Custom Field types in § Usage, fixing `owner-email`
  Project-type => Issue-type

Peace,
tiredpixel