[x/tour] content: add note about using rand.Seed with the time
[x/tour] content: add note about using rand.Seed with the time

Fixes golang/tour#12

Change-Id: I3e31796e68052c424ef6b5670aac516e461d1809
Reviewed-on: https://go-review.googlesource.com/44931
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
X-Tour-Commit: d844dc7a157c5d2e92bccc9732530c37bc7ef33b

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website