Fix missing dc: prefix in agentGroup example

my implementation at https://github.com/inrupt/wac-ldp choked on this when I tried to use it as a test fixture :)

Author: michielbdejong

README.md

@@ -264,6 +264,7 @@ Corresponding `work-groups` Group Listing document:
 ```ttl
 # Contents of https://alice.example.com/work-groups
 @prefix    acl:  <http://www.w3.org/ns/auth/acl#>.
+@prefix     dc:  <http://purl.org/dc/elements/1.1/>.
 @prefix  vcard:  <http://www.w3.org/2006/vcard/ns#>.
 
 <#Accounting>