Remove "not implemented" note for Entries, fix typos, update Expenses

* The Entries are now implemented in API v2, so the note about them
  not being implemented needs to be removed.
* @timcraft pointed out a typo in https://github.com/cheerful/freckle-apidocs/commit/9a980cd3661ca939565424f421c1633777d5673c#commitcomment-7161981
  which has been fixed
  * The query parameter examples for Expenses have been updated too
* When searching for expenses you will be able to filter by whether
  it is invoiced or uninvoiced.