net/url: update docs to match renamed receiver

The receiver was renamed 6 years ago in https://golang.org/cl/5674065
but the docs weren't updated to match.

Change-Id: I5e72cedc0e0f067382545d272f48a9c7dfb5a9b7
Reviewed-on: https://go-review.googlesource.com/104116Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/url/url.go

@@ -952,7 +952,7 @@ func (u *URL) Parse(ref string) (*URL, error) {
 }
 
 // ResolveReference resolves a URI reference to an absolute URI from
-// an absolute base URI, per RFC 3986 Section 5.2.  The URI reference
+// an absolute base URI u, per RFC 3986 Section 5.2. The URI reference
 // may be relative or absolute. ResolveReference always returns a new
 // URL instance, even if the returned URL is identical to either the
 // base or reference. If ref is an absolute URL, then ResolveReference