playground: fix share URL

The baseURL is used to construct the target URLs for both /compile
and /share. When I changed it recently, I forgot about the latter case.

Change-Id: Icde617b1cc3d31d98ca4b62e313b9d886253f1cc
Reviewed-on: https://go-review.googlesource.com/22689
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed