Update readme for headers names

Author: ash1day

README.md

@@ -325,9 +325,15 @@ module YourApp
       allow do
         origins '*'
         resource '*',
-          :headers => :any,
-          :expose  => ['access-token', 'expiry', 'token-type', 'uid', 'client'],
-          :methods => [:get, :post, :options, :delete, :put]
+          headers: :any,
+          expose: [
+            DeviseTokenAuth.headers_names[:'access-token'],
+            DeviseTokenAuth.headers_names[:'client'],
+            DeviseTokenAuth.headers_names[:'expiry'],
+            DeviseTokenAuth.headers_names[:'uid'],
+            DeviseTokenAuth.headers_names[:'token-type']
+          ],
+          methods: [:get, :post, :options, :delete, :put]
       end
     end
   end