godoc: correct fsRoot to prevent misleading message about cmd/

Fixes golang/go#14447

GitHub-Last-Rev: 98ea70ca4df4e1bdead0b3960ef0381b4f81572b
GitHub-Pull-Request: golang/tools#24
Change-Id: Ida7db414a47ea6f6da5de400bcb6a5aea03f1d05
Reviewed-on: https://go-review.googlesource.com/94841
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/pres.go

@@ -116,10 +116,11 @@ func NewPresentation(c *Corpus) *Presentation {
 		},
 	}
 	p.cmdHandler = handlerServer{
-		p:       p,
-		c:       c,
-		pattern: "/cmd/",
-		fsRoot:  "/src",
+		p:           p,
+		c:           c,
+		pattern:     "/cmd/",
+		stripPrefix: "cmd/",
+		fsRoot:      "/src/cmd",
 	}
 	p.pkgHandler = handlerServer{
 		p:           p,