install webrick in readme

Change-Id: Ib3f4217304cae34f15681455d90fe71b134591e1

Author: zhuzhenfeng.code

README.md

@@ -9,6 +9,7 @@ Install `github-pages` (requires Ruby)
 
 ```sh
 gem install github-pages
+gem install webrick # For Ruby3.x
 ```
 
 and run `make serve` in root directory.