net/url: correct documentation of Values.Add

Fixes #7816.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/98160043

src/pkg/net/url/url.go

@@ -502,7 +502,7 @@ func (v Values) Set(key, value string) {
 	v[key] = []string{value}
 }
 
-// Add adds the key to value. It appends to any existing
+// Add adds the value to key. It appends to any existing
 // values associated with key.
 func (v Values) Add(key, value string) {
 	v[key] = append(v[key], value)