scripts/coccinelle: fix typos

Fix a couple spelling mistakes.
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NMichal Marek <mmarek@suse.cz>

scripts/coccinelle/null/deref_null.cocci

 ///
-/// A variable is dereference under a NULL test.
-/// Even though it is know to be NULL.
+/// A variable is dereferenced under a NULL test.
+/// Even though it is known to be NULL.
 ///
 // Confidence: Moderate
 // Copyright: (C) 2010 Nicolas Palix, DIKU.  GPLv2.