doc/gopher: fix typo in README

LGTM=josharian
R=golang-codereviews, josharian
CC=golang-codereviews
https://golang.org/cl/80220043

doc/gopher/README

 The Go gopher was designed by Renee French. (http://reneefrench.blogspot.com/)
-The design is licsensed under the Creative Commons 3.0 Attributions license.
+The design is licensed under the Creative Commons 3.0 Attributions license.
 Read this article for more details: http://blog.golang.org/gopher
-