Fix void field slot

This shouldn't cause broken generated code, but it did.

--- a/aircraftlib/aircraft.capnp
+++ b/aircraftlib/aircraft.capnp
@@ -68,7 +68,7 @@ struct Aircraft {
   #  certain places.
 
   union {
-    void      @0: Void; # @0 will be the default, so always make @0 a Void.
+    foo       @0: Void; # @0 will be the default, so always make @0 a Void.
     b737      @1: B737;
     a320      @2: A320;
     f16       @3: F16;