Update webpack requirement in /packages/generator/app/templates

Updates the requirements on [webpack](https://github.com/webpack/webpack) to permit the latest version.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](webpack/webpack@v4.39.2...v4.39.3)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

packages/generator/app/templates/package.json

@@ -40,7 +40,7 @@
     "react": "^16.9.0",
     "react-dom": "^16.9.0",
     "redbox-react": "^1.6.0",
-    "webpack": "^4.39.2",
+    "webpack": "^4.39.3",
     "webpack-cli": "^3.3.7",
     "webpack-dev-server": "^3.8.0"
   },