Fix the plurality header docs for v0.4 (#32)

admin.rst

@@ -248,7 +248,7 @@ As discussed in `Singular or Plural`_, there are no special URL forms for singul
 .. code:: http
 
   GET /people?id=eq.1
-  Prefer: plurality=singular
+  Accept: application/vnd.pgrst.object+json
 
 This allows compound primary keys and makes the intent for singular response independent of a URL convention. However for any table which uses a simple primary key you can use Nginx to simulate the familiar URL convention.
 

api.rst

@@ -218,12 +218,12 @@ By default PostgREST returns all JSON results in an array, even when there is on
     { "id": 1 }
   ]
 
-This can be inconvenient for client code. To return the first result as an object unenclosed by an array, Include a Prefer request header
+This can be inconvenient for client code. To return the first result as an object unenclosed by an array, specify :code:`vnd.pgrst.object` as part of the :code:`Accept` header
 
 .. code:: http
 
   GET /items?id=eq.1 HTTP/1.1
-  Prefer: plurality=singular
+  Accept: application/vnd.pgrst.object+json
 
 This returns