Wrong classes in default custom-theme.yml

Unfortunately, asciidoctor changed some class names.
The following lines in the default yaml theme don't work any more:

```
footer:
height: 0.75in
line_height: 1
recto_content:
right: '{chapter-title} | *{page-number}*'
verso_content:
left: '*{page-number}* | {chapter-title}'
```
This is how it should look like:
```
footer:
height: 0.75in
line_height: 1
recto:
right:
content: '{chapter-title} | *{page-number}*'
verso:
left:
content: '*{page-number}* | {chapter-title}'
```
Just in case anyone else is having trouble.
Update: Same applies to the header.