Doc fix for the Builtin.Executor mangling

Author: rjmccall

docs/ABI/Mangling.rst

@@ -516,7 +516,7 @@ Types
   type ::= 'BB'                              // Builtin.UnsafeValueBuffer
   type ::= 'Bc'                              // Builtin.RawUnsafeContinuation
   type ::= 'BD'                              // Builtin.DefaultActorStorage
-  type ::= 'Be'                              // Builtin.ExecutorRef
+  type ::= 'Be'                              // Builtin.Executor
   type ::= 'Bf' NATURAL '_'                  // Builtin.Float<n>
   type ::= 'Bi' NATURAL '_'                  // Builtin.Int<n>
   type ::= 'BI'                              // Builtin.IntLiteral