README: pull gopher image from website
README: pull gopher image from website

Fixes breakage accidentally introduced by https://golang.org/cl/291711.

Fixes #44295

Change-Id: I76f3e5577d1d24027d4ed2a725b5b749ab2d059c
Reviewed-on: https://go-review.googlesource.com/c/go/+/292629
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go