net/http: fix parameter name in comment

Fixes #21077

Change-Id: Ic61d7313907f58ff4027fd2eee1ddb8c1656304d
Reviewed-on: https://go-review.googlesource.com/49712Reviewed-by: Dmitri Shuralyov <shurcool@gmail.com>

src/net/http/server.go

@@ -1958,7 +1958,7 @@ func StripPrefix(prefix string, h Handler) Handler {
 	})
 }
 
-// Redirect replies to the request with a redirect to url,
+// Redirect replies to the request with a redirect to urlStr,
 // which may be a path relative to the request path.
 //
 // The provided code should be in the 3xx range and is usually