re-added rshift to KEYEVTS, no clue how they got lost; Closes PR12500; please merge into 5.2.1

collects/2htdp/universe.rkt

@@ -211,6 +211,7 @@
     "cancel"
     "clear"
     "shift"
+    "rshift"
     "control"
     "menu"
     "pause"

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

@@ -298,6 +298,7 @@ Second, some keys have multiple-character string representations. Strings
 @item{@racket["cancel"]}
 @item{@racket["clear"]}
 @item{@racket["shift"]}
+@item{@racket["rshift"]}
 @item{@racket["control"]}
 @item{@racket["menu"]}
 @item{@racket["pause"]}