content: make description about rand.Intn less ambiguous

Technically rand.Intn will always return the same sequence of
numbers, however in this context, re-running the example will
always produce the same value.

Fixes golang/go#12370

Change-Id: Iea87a4b1b0db1a919a35b00d0d4af497fafd1319
Reviewed-on: https://go-review.googlesource.com/15555
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed