explicitly close evaluator for plot doc

Although this shouldn't affect document rendering, since
each document is run under a custodian that is shut down,
it simplifies using the document individually to check for
leaks.

collects/plot/scribblings/common.rkt

@@ -38,3 +38,6 @@
                                  [plot3d-bitmap  plot3d])
                       plot/utils)))
     eval))
+
+(define (close-plot-eval)
+  (close-eval plot-eval))

collects/plot/scribblings/plot.scrbl

@@ -46,3 +46,5 @@ If you have code written for PLoT 5.1.3 or earlier, please see @secref["porting"
 @include-section["porting.scrbl"]
 
 @include-section["compat.scrbl"]
+
+@close-plot-eval[]