doc: spelling error changes

Fixed multiple spelling errors.

Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Carlos E. Garcia <carlos@cgarcia.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/fb/sstfb.txt b/Documentation/fb/sstfb.txt
index 550ca77..13db107 100644
--- a/Documentation/fb/sstfb.txt
+++ b/Documentation/fb/sstfb.txt
@@ -10,7 +10,7 @@
 	  The main page is located at <http://sstfb.sourceforge.net>, and if
 	you want the latest version, check out the CVS, as the driver is a work
 	in progress, I feel uncomfortable with releasing tarballs of something
-	not completely working...Don't worry, it's still more than useable
+	not completely working...Don't worry, it's still more than usable
 	(I eat my own dog food)
 
 	  Please read the Bug section, and report any success or failure to me