Revert "godoc: correct fsRoot to prevent misleading message about cmd/"

This reverts commit db9df82880.

Reason for revert: Broke the build. I forgot to run tests, sorry.

Change-Id: I8a6ef88cac2aa638982530960f0d37a32dc5ccf0
Reviewed-on: https://go-review.googlesource.com/95875
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/pres.go

@@ -119,8 +119,7 @@ func NewPresentation(c *Corpus) *Presentation {
 		p:       p,
 		c:       c,
 		pattern: "/cmd/",
-		stripPrefix: "cmd/",
-		fsRoot:      "/src/cmd",
+		fsRoot:  "/src",
 	}
 	p.pkgHandler = handlerServer{
 		p:           p,