internal/blog: fix atom feed self link

Fixes golang/go#49595

Change-Id: Ie28221bf3dfe804ce3124040b15569a63ace56fa
Reviewed-on: https://go-review.googlesource.com/c/website/+/364275
Trust: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/blog/blog.go

@@ -40,7 +40,7 @@ func atomFeed(site *web.Site) ([]byte, error) {
 		Updated: atom.Time(updated),
 		Link: []atom.Link{{
 			Rel:  "self",
-			Href: baseURL + "/feed.atom",
+			Href: baseURL + "/blog/feed.atom",
 		}},
 	}