Fixed a typo in the name of the "outter" property

As reported in the issue #129 the correct English word is "outer" instead
of "outter".

easybook now promotes the use of the "outer" option, but the previous "outter"
option still works as expected. Therefore, this change is backwards compatible
thanks to the default() filter of Twig:

{{ edition.margin.outer|default(edition.margin.outter)|default('20mm') }}