content/go-brand: make slide theme license match all our other content

The blog post says the slide theme is licensed under
Creative Commons Attribution-ShareAlike 4.0 International License
but we license all our blog content and the gopher images
under Creative Commons Attributions 3.0.
Do the same here, after discussion with Steve Francia.
The use of CC-SA 4 was an oversight.

Change-Id: I0bfa720627be719107636f483aca01ba94b229a4
Reviewed-on: https://go-review.googlesource.com/109916
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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