README: poac.dev -> cabinpkg.com

Author: ken-matsui

README.md

@@ -1,6 +1,6 @@
 ## Website
 
-[![SecurityHeaders.io](https://securityheadersiobadges.azurewebsites.net/create/badge?domain=https://poac.dev)](https://securityheaders.io/?q=https://poac.dev&hide=on&followRedirects=on)
+[![SecurityHeaders.io](https://securityheadersiobadges.azurewebsites.net/create/badge?domain=https://cabinpkg.com)](https://securityheaders.io/?q=https://cabinpkg.com&hide=on&followRedirects=on)
 
 A package registry website for Cabin, a package manager and build system for C++.