net/tshttpproxy: fix typo in Windows code added at least second

I ran tests & vet & staticcheck, but for Linux, not Windows.
(#728 would be nice)

net/tshttpproxy/tshttpproxy_windows.go

@@ -63,7 +63,7 @@ func proxyFromWinHTTPOrCache(req *http.Request) (*url.URL, error) {
 		if err == nil {
 			cachedProxy.Lock()
 			defer cachedProxy.Unlock()
-			if was, now := fmt.Sprint(cachedProxy.val), fmt.Sprint(res.proxy); want != now {
+			if was, now := fmt.Sprint(cachedProxy.val), fmt.Sprint(res.proxy); was != now {
 				log.Printf("tshttpproxy: winhttp: updating cached proxy setting from %v to %v", was, now)
 			}
 			cachedProxy.val = res.proxy