Update links to the WG (#871)

README.md

@@ -1,8 +1,8 @@
 This repository is the home of the :star: **[Web App Manifest](https://www.w3.org/TR/appmanifest/)** :star: specification being worked on by
-the [Web Platform Working Group](https://www.w3.org/WebPlatform/WG/).
+the [Web Applications Working Group](https://www.w3.org/2019/webapps/).
 
 ## Useful links
 * [Explainer](https://github.com/w3c/manifest/blob/gh-pages/explainer.md)
 * [The Web App Manifest specification](https://www.w3.org/TR/appmanifest/)
 * [Use cases and requirements](https://w3c-webmob.github.io/installable-webapps/)
-* [The Web Platform WG homepage](https://www.w3.org/WebPlatform/WG/)
+* [The Web Applications WG homepage](https://www.w3.org/2019/webapps/)

index.html

@@ -3068,8 +3068,8 @@ <h3>
             Person & email address to contact for further information:
           </dt>
           <dd>
-            The <a href="https://www.w3.org/WebPlatform/WG/" rel="nofollow">Web
-            Platform Working Group</a> can be contacted at <a href=
+            The <a href="https://www.w3.org/2019/webapps/" rel="nofollow">Web
+            Applications Working Group</a> can be contacted at <a href=
             "https://lists.w3.org/Archives/Public/public-webapps/" rel=
             "nofollow">public-webapps@w3.org</a>.
           </dd>
@@ -3089,7 +3089,7 @@ <h3>
             Author:
           </dt>
           <dd>
-            W3C's Web Platform Working Group.
+            W3C's Web Applications Working Group.
           </dd>
           <dt>
             Change controller: