Update comment making reference to an old file.

The monitor layout is described in monitor.cc. A comment was making reference
to a non exisitng Sync.c file. This CL updates this comment.

Change-Id: I1b3523827dc081b15d077ac50af601a72ed52d5d
diff --git a/src/compiler/codegen/arm/call_arm.cc b/src/compiler/codegen/arm/call_arm.cc
index 225c334..e8dc10b 100644
--- a/src/compiler/codegen/arm/call_arm.cc
+++ b/src/compiler/codegen/arm/call_arm.cc
@@ -468,7 +468,7 @@
  *
  * The result of the strex is 0 if we acquire the lock.
  *
- * See comments in Sync.c for the layout of the lock word.
+ * See comments in monitor.cc for the layout of the lock word.
  * Of particular interest to this code is the test for the
  * simple case - which we handle inline.  For monitor enter, the
  * simple case is thin lock, held by no-one.  For monitor exit,