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>

Documentation/doc-guide/sphinx.rst

@@ -147,7 +147,7 @@ By default, the build will try to use the Read the Docs sphinx theme:
 
 If the theme is not available, it will fall-back to the classic one.
 
-The Sphinx theme can be overriden by using the ``DOCS_THEME`` make variable.
+The Sphinx theme can be overridden by using the ``DOCS_THEME`` make variable.
 
 To remove the generated documentation, run ``make cleandocs``.