Fix doctest for MarkupLM
# What does this PR do?

The CI complains
```
UNEXPECTED EXCEPTION: SyntaxError('EOF while scanning triple-quoted string literal', ('<doctest markuplm.mdx[2]>', 7, 96, 'html_string = """\n <!DOCTYPE html>\n <html>\n <head>\n <title>Hello world</title>\n </head>\n <body>\n'))
```
due to the lack of `...` in some empty lines. But `make style` will remove those `...` after I add it to those lines.

So I just removed those empty lines to pass the doctest.
_The documentation is not available anymore as the PR was closed or merged._