log/slog: correct doc examples for Handler.WithGroup

[callthingsoff]

Fixes #64519

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Message from Jes Cok:

Patch Set 1: Run-TryBot+1

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Gopher Robot:

Patch Set 1: TryBot-Result+1

(1 comment)

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Message from Jonathan Amsterdam:

Patch Set 1: Code-Review+2

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Message from Jonathan Amsterdam:

Patch Set 1:

(1 comment)

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Message from Bryan Mills:

Patch Set 1: Code-Review+1

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Message from Jes Cok:

Patch Set 1:

(1 comment)

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Message from Jonathan Amsterdam:

Patch Set 1:

(1 comment)

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This PR is being closed because golang.org/cl/547215 has been merged.