Fix the checkbox to disable the Welcome page

Recently, we reimplemented the Welcome page in React. Unfortunately, we
broke the checkbox that enables/disables the Welcome page and it would
allow checking but wouldn't allow unchecking.

react/features/welcome/components/WelcomePage.web.js

      */
     _onDisableWelcomeChange(event) {
         this.setState({
-            enableWelcomePage: !event.target.value
+            enableWelcomePage: !event.target.checked
         }, () => {
             APP.settings.setWelcomePageEnabled(this.state.enableWelcomePage);
         });