Minor fixes to kernel-doc.rst

The author clearly meant to use the word 'which' here.  Also replace
some tabs with spaces which fixes the syntax highlighting in my editor.
Signed-off-by: NMatthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/doc-guide/kernel-doc.rst

@@ -277,7 +277,7 @@ named ``Return``.
 
   #) If the descriptive text you provide has lines that begin with
      some phrase followed by a colon, each of those phrases will be taken
-     as a new section heading, with probably won't produce the desired
+     as a new section heading, which probably won't produce the desired
      effect.
 
 Structure, union, and enumeration documentation