added rcontrol to key events and docs; robby please test

(cherry picked from commit 1d9d0401df)

collects/2htdp/universe.rkt

@@ -213,6 +213,7 @@
     "shift"
     "rshift"
     "control"
+    "rcontrol"
     "menu"
     "pause"
     "capital"

collects/teachpack/2htdp/scribblings/universe.scrbl

@@ -300,6 +300,7 @@ Second, some keys have multiple-character string representations. Strings
 @item{@racket["shift"]}
 @item{@racket["rshift"]}
 @item{@racket["control"]}
+@item{@racket["rcontrol"]}
 @item{@racket["menu"]}
 @item{@racket["pause"]}
 @item{@racket["capital"]}