[Kernel] Fix specification of constant reference.

Change-Id: Ic17294da65e3f27d52692975a48b8aa3132c2739
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/100841
Commit-Queue: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>
Auto-Submit: Aske Simon Christensen <askesc@google.com>
Reviewed-by: Kevin Millikin <kmillikin@google.com>

pkg/kernel/binary.md

@@ -87,7 +87,7 @@ type StringReference {
 }
 
 type ConstantReference {
-  UInt index; // Index into the Component's constants.
+  UInt offset; // Byte offset into the Component's constants.
 }
 
 type SourceInfo {