tour: playground is always on a Tuesday

[crisman]

In "tour/flowcontrol/10" ("Switch evaluation order") the interaction between the code using "Now()" and the Go Playground starting on 2009-11-10 is less than obvious. The data the reader needs is that it starts on a Tuesday.

Add the day of week to the online Go Playground note for readers to be able to understand the output.

Fixes golang/tour#1407 Fixes golang/tour#1339 Fixes golang/tour#1249 Fixes golang/tour#1068

[gopherbot]

This PR (HEAD: ca06872062f95e4202a9c38e497d59241a3a075d) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/473575 to see it.

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