Update tests to reflect change of web app manifest extension in React C# template

See dotnet/spa-templates#80

Author: christianliebel

src/ProjectTemplates/test/Templates.Tests/template-baselines.json

@@ -1684,7 +1684,7 @@
       "Files": [
         "ClientApp/public/favicon.ico",
         "ClientApp/public/index.html",
-        "ClientApp/public/manifest.json",
+        "ClientApp/public/manifest.webmanifest",
         "ClientApp/src/components/Counter.js",
         "ClientApp/src/components/FetchData.js",
         "ClientApp/src/components/Home.js",