Correcting dictionary entry for `create-temporary-plot-pen`

Looks to me like `create-temporary-plot-pen` has been working this way since when the code was first checked into GitHub more than 3 years ago.

See [here](https://github.com/NetLogo/NetLogo/blob/4fbfaff024e04b95eb08a83137e8ad5d7cc5df46/src/main/org/nlogo/prim/plot/_plotprims.scala#L86) for the code in the original commit.  No runtime exception to be seen.