net/http: fix typo in comment

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/138960043

src/pkg/net/http/fs.go

@@ -403,7 +403,7 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 		return
 	}
 
-	// serverContent will check modification time
+	// serveContent will check modification time
 	sizeFunc := func() (int64, error) { return d.Size(), nil }
 	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
 }