[x/tour] tour: add link to previous sqrt exercise
[x/tour] tour: add link to previous sqrt exercise

Change-Id: Ib3fcc8c1bec38b7b9fae426d61161e1ce2982ded
Reviewed-on: https://go-review.googlesource.com/12718
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
X-Tour-Commit: 011b65dab99c7dfc540d2a51db230a76536d8399

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