docs: Kconfig: Fix help text indentation

The convention for help text indentation seems to be a single tab followed
by two whitespaces. Do that.

While at it, remove a stray empty line.
Signed-off-by: NJuerg Haefliger <juergh@canonical.com>
Link: https://lore.kernel.org/r/20220517140837.331298-1-juergh@canonical.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/Kconfig

 config WARN_MISSING_DOCUMENTS
-
 	bool "Warn if there's a missing documentation file"
 	depends on COMPILE_TEST
 	help