net/url: fix docs for URL.String

Fixes #10227.

Change-Id: I64d5522e76da5a717e3c4169405e5ef35d6c262e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7974Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/url/url.go

@@ -443,7 +443,7 @@ func parseAuthority(authority string) (user *Userinfo, host string, err error) {
 // String reassembles the URL into a valid URL string.
 // The general form of the result is one of:
 //
-//	scheme:opaque
+//	scheme:opaque?query#fragment
 //	scheme://userinfo@host/path?query#fragment
 //
 // If u.Opaque is non-empty, String uses the first form;