[PATCH] Trivial spelling fix in ppc code comments

[Sankar P]

Fixes a trivial spelling error in powerpc code comments. 

Signed-off-by: Sankar P <sankar.curiosity at gmail.com>

---
diff --git a/arch/powerpc/mm/slb.c b/arch/powerpc/mm/slb.c
index 89497fb..3b52c80 100644
--- a/arch/powerpc/mm/slb.c
+++ b/arch/powerpc/mm/slb.c
@@ -2,7 +2,7 @@
  * PowerPC64 SLB support.
  *
  * Copyright (C) 2004 David Gibson <dwg at au.ibm.com>, IBM
- * Based on earlier code writteh by:
+ * Based on earlier code written by:
  * Dave Engebretsen and Mike Corrigan {engebret|mikejc}@us.ibm.com
  *    Copyright (c) 2001 Dave Engebretsen
  * Copyright (C) 2002 Anton Blanchard <anton at au.ibm.com>, IBM