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/usb/mass-storage.txt b/Documentation/usb/mass-storage.txt
index 59063ad..e89803a 100644
--- a/Documentation/usb/mass-storage.txt
+++ b/Documentation/usb/mass-storage.txt
@@ -13,7 +13,7 @@
   operation.
 
   Note that the driver is slightly non-portable in that it assumes
-  a single memory/DMA buffer will be useable for bulk-in and bulk-out
+  a single memory/DMA buffer will be usable for bulk-in and bulk-out
   endpoints.  With most device controllers this is not an issue, but
   there may be some with hardware restrictions that prevent a buffer
   from being used by more than one endpoint.