doc/play: revise peano.go comment again.

Change-Id: I412358e99301d1c9603960fd34601dae60fca3cb
Reviewed-on: https://go-review.googlesource.com/1400Reviewed-by: Rob Pike <r@golang.org>

doc/play/peano.go

@@ -3,9 +3,9 @@
 // (the nodes are the data).
 // http://en.wikipedia.org/wiki/Peano_axioms
 
-// This program demonstrates the effectiveness
-// of the Go runtime's dynamically growing
-// stacks for heavily recursive computations.
+// This program demonstrates that Go's automatic
+// stack management can handle heavily recursive
+// computations.
 
 package main