[x/tour] content: clean up slice of slices code example
[x/tour] content: clean up slice of slices code example

The variable represents a board, not a game.

Also, there's no point in creating a helper function to print
the board, since it's only printed once.

Change-Id: I34897302d80d0aa8de63a424f570c39f64fa6538
Reviewed-on: https://go-review.googlesource.com/19400
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
X-Tour-Commit: 1631c208f723828abbecbd92bdceb4000b0042e1

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