Use HTTPS by default in the webview sample

This allows the sample to load the initial page when built using macOS
10.11 or later SDK which enables "app transport security" (ATS), which
prevents HTTP connections by default.

samples/webview/webview.cpp

@@ -66,7 +66,7 @@ class WebApp : public wxApp
 {
 public:
     WebApp() :
-        m_url("http://www.wxwidgets.org")
+        m_url("https://www.wxwidgets.org")
     {
     }