publish.yml: Bump an example version a little

3.x is getting old. I wanted to at least see 4.x.

Author: k0kubun

.github/workflows/publish.yml

@@ -9,7 +9,7 @@ on:
       version:
         description: 'Version of the Ruby package to release'
         required: true
-        default: '3.3.4'
+        default: '4.0.0'
 
 permissions:
   contents: read