blog: fix link to cgo source file in Cgo post.

Change-Id: Id22ff1842f85ceb312c2f5e37e6440e7e80ac727
Reviewed-on: https://go-review.googlesource.com/15690
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed