man: fix list of escaped characters in unit names

The code works differently than the docs, and the code is right here.
Fix the doc hence.

See VALID_CHARS in unit-name.c for details about allowed chars in unit
names, but keep in mind that "-" and "\" are special, since generated by
the escaping logic: they are OK to show up in unit names, but need to be
escaped when converting foreign strings to unit names to make sure
things remain reversible.

Fixes: #19623