Add "homepage" key to package.json

Setting the "homepage" key to package.json let the create-react-app uses
the value as the application's root URI. By setting a URI which contains
a subdirectory in it will make URIs for static resources will be ones
relative to that subdirectory.

package.json

@@ -2,6 +2,7 @@
   "name": "ex-antd-app",
   "version": "0.1.0",
   "private": true,
+  "homepage": "https://sgryjp.github.com/ex-antd-app",
   "dependencies": {
     "@testing-library/jest-dom": "^4.2.4",
     "@testing-library/react": "^9.4.0",