Fix typos in Documentation/: 'F'-'G'

This patch fixes typos in various Documentation txts. The patch addresses
some words starting with the letters 'F'-'G'.

Signed-off-by: Matt LaPlante <kernel1@cyberdogtech.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
diff --git a/Documentation/block/barrier.txt b/Documentation/block/barrier.txt
index 761073e..de3d88e 100644
--- a/Documentation/block/barrier.txt
+++ b/Documentation/block/barrier.txt
@@ -56,7 +56,7 @@
 i. No write-back cache.  Keeping requests ordered is enough.
 
 ii. Write-back cache but no flush operation.  There's no way to
-gurantee physical-medium commit order.  This kind of devices can't to
+guarantee physical-medium commit order.  This kind of devices can't to
 I/O barriers.
 
 iii. Write-back cache and flush operation but no FUA (forced unit