README: pull gopher image from website

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

Fixes golang#44295

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

README.md

@@ -3,7 +3,7 @@
 Go is an open source programming language that makes it easy to build simple,
 reliable, and efficient software.
 
-![Gopher image](doc/gopher/fiveyears.jpg)
+![Gopher image](https://golang.org/doc/gopher/fiveyears.jpg)
 *Gopher image by [Renee French][rf], licensed under [Creative Commons 3.0 Attributions license][cc3-by].*
 
 Our canonical Git repository is located at https://go.googlesource.com/go.