Documentation: bcachefs: casefolding: Do not italicize NUL

Documentation: bcachefs: casefolding: Do not italicize NUL

Sphinx reports htmldocs warning:

Documentation/filesystems/bcachefs/casefolding.rst:36: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]

That's because NUL word is italicized but it is written in plural form
instead (`NUL`s). Sphinx, however, doesn't tip over when the italicized
word in this fashion is followed by punctuation instead.

Do not italicize the word to keep Sphinx happy.

Fixes: bc5cc09246c5 ("bcachefs: bcachefs_metadata_version_casefolding")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250221162135.79be0147@canb.auug.org.au/
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>