docs: address some text issues with css/theme support

Fix:
- overriden ->overridden
- some whitespace issues introduced at the css/theme
  Makefile help.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/b0b166025019f7cc4f122bd789c79ba28cc2d29d.1639212812.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>