tour: link to time doc for After/Tick #219
                
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On the tour slide "tour/concurrency/6" ("Default Selection") the example code in "default-selection.go" uses time.After() and time.Tick() without any mention of what they do. A link to the time fuctions doc can provide context to understand what those do.
Add link to time#pkg-functions on slide next to code using time.After()/Tick().
Fixes golang/tour#1466