Fixed an error in the PHPDoc of a function
Books can now use their own cover when published as PDF

Similarly to ePub books, to define your own PDF cover, create
a file named `cover.pdf` and place it in any of the following paths
(the paths are shown by priority, so the cover will be the first PDF
file found):

1. <book>/Resources/Templates/<edition-name>/cover.pdf
2. <book>/Resources/Templates/<edition-format>/cover.pdf
3. <book>/Resources/Templates/cover.pdf