context: add default to select example in comment of Done

[davidhsingyuchen]

According to the comment of Stream, it should return once ctx.Done is
closed, but the current implementation may select another case instead
if out is ready to receive an element.

Quoting from the related Go spec: "If one or more of the
communications can proceed, a single one that can proceed is chosen
via a uniform pseudo-random selection."

https://go.dev/ref/spec#Select_statements

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Patch Set 1:

(1 comment)

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Message from Hsing-Yu Chen:

Patch Set 2:

(1 comment)

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Message from Damien Neil:

Patch Set 2:

(1 comment)

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Message from Hsing-Yu Chen:

Patch Set 2:

(1 comment)

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