fix some typos in the doc

llvm-svn: 292014
diff --git a/llvm/docs/WritingAnLLVMBackend.rst b/llvm/docs/WritingAnLLVMBackend.rst
index f06a95d..0fc7382 100644
--- a/llvm/docs/WritingAnLLVMBackend.rst
+++ b/llvm/docs/WritingAnLLVMBackend.rst
@@ -593,12 +593,12 @@
     FPRegsClass     FPRegsRegClass;
     IntRegsClass    IntRegsRegClass;
   ...
-    // IntRegs Sub-register Classess...
+    // IntRegs Sub-register Classes...
     static const TargetRegisterClass* const IntRegsSubRegClasses [] = {
       NULL
     };
   ...
-    // IntRegs Super-register Classess...
+    // IntRegs Super-register Classes..
     static const TargetRegisterClass* const IntRegsSuperRegClasses [] = {
       NULL
     };