time: add basic YYYY/MM/DD example to time docs

time: add basic YYYY/MM/DD example to time docs

This is a _very_ common question [1]. Let's just make an example for it.

1: https://www.google.com/search?q=golang+yyyy-mm-dd&oq=golang+yyyy-mm-dd&aqs=chrome..69i57j0l4j69i64l3.6015j0j7&sourceid=chrome&ie=UTF-8

Change-Id: I32ae689b91018d326f31a2442a1beaf68dddf13c
Reviewed-on: https://go-review.googlesource.com/c/go/+/220595
Run-TryBot: Jean de Klerk <deklerk@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>