update user-guide url

Our user-guide is now properly hosted at https://book.metal3.io/

SECURITY_CONTACTS

@@ -7,7 +7,7 @@ Please do:
 
 In this repository security reports are handled according to the
 Metal3-io project's security policy. For more information about the security
-policy consult the User-Guide [here](https://metal3io.netlify.app/security_policy.html).
+policy consult the User-Guide [here](https://book.metal3.io/security_policy.html).
 
 As this repository has a rolling release cycle and there are no release
 branches, the security fix will be merged to the main branch. The fix will