Fix spello

llvm-svn: 8432
diff --git a/llvm/include/Support/Casting.h b/llvm/include/Support/Casting.h
index a5a0955..2815ecd 100644
--- a/llvm/include/Support/Casting.h
+++ b/llvm/include/Support/Casting.h
@@ -62,7 +62,7 @@
 };
 
 // isa_impl_cl - Use class partial specialization to transform types to a single
-// cannonical form for isa_impl.
+// canonical form for isa_impl.
 //
 template<typename FromCl>
 struct isa_impl_cl {