[optimizing] Address x86_64 RIP patch comments

Nicolas had some comments after the patch
https://android-review.googlesource.com/#/c/144100 had merged.  Fix the
problems that he found.

Change-Id: I40e8a4273997860db7511dc8f1986281b72bead2
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>
diff --git a/compiler/optimizing/code_generator_x86_64.h b/compiler/optimizing/code_generator_x86_64.h
index c819eec..aae7de0 100644
--- a/compiler/optimizing/code_generator_x86_64.h
+++ b/compiler/optimizing/code_generator_x86_64.h
@@ -297,7 +297,7 @@
   X86_64Assembler assembler_;
   const X86_64InstructionSetFeatures& isa_features_;
 
-  // Offset to start of the constant area in the assembled code.
+  // Offset to the start of the constant area in the assembled code.
   // Used for fixups to the constant area.
   int constant_area_start_;