fix the :help docs for :set stop (#2737)

compiler/ghci/InteractiveUI.hs

@@ -255,7 +255,7 @@ helpText =
  "   :set prog <progname>        set the value returned by System.getProgName\n" ++
  "   :set prompt <prompt>        set the prompt used in GHCi\n" ++
  "   :set editor <cmd>           set the command used for :edit\n" ++
- "   :set stop <cmd>             set the command to run when a breakpoint is hit\n" ++
+ "   :set stop [<n>] <cmd>       set the command to run when a breakpoint is hit\n" ++
  "   :unset <option> ...         unset options\n" ++
  "\n" ++
  "  Options for ':set' and ':unset':\n" ++