README: Poac -> Cabin

Author: ken-matsui

README.md

@@ -2,7 +2,7 @@
 
 [![SecurityHeaders.io](https://securityheadersiobadges.azurewebsites.net/create/badge?domain=https://poac.dev)](https://securityheaders.io/?q=https://poac.dev&hide=on&followRedirects=on)
 
-A package registry website for Poac, a package manager and build system for C++.
+A package registry website for Cabin, a package manager and build system for C++.
 
 ### Contributing