Say who should enforcement mechanism. (#433)

index.html

@@ -1234,7 +1234,7 @@
 privacy principles published, for more specific [=contexts=] on the web.
 
 These principles should be enforced by [=user agents=]. When this is not possible,
-additional enforcement mechanisms are needed.
+we encourage other entities to find ways to enforce them.
 
 ## Identity on the Web {#identity}