Gopher.md: update gopher license version

Updates the wiki to match other places the gopher license is mentioned.

CL 447156 updated the Gopher image license to CC-BY-4.0. CL 588995
updated a missed mention inside the website repository, so it seems that
updating the license in other places is desirable.

Change-Id: I597ef45a03f853b8d635207effcc001ef8b6f077
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/651976
Reviewed-by: Ian Lance Taylor <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: dagood

Gopher.md

@@ -3,7 +3,7 @@ title: Gopher
 ---
 
 The Go gopher was designed by [Renee French](https://www.instagram.com/reneefrench/).  
-The design is licensed under the Creative Commons 3.0 Attributions license.  
+The design is licensed under the Creative Commons 4.0 Attributions license.  
 Read [this article](https://go.dev/blog/gopher) and watch [The Go Gopher A Character Study](https://www.youtube.com/watch?v=4rw_B4yY69k) for more details.
 
 ## Where can I find more gopher illustrations?