Align coding style

The last instantiation of call had mismatched showing filter's return value with the two previous instantiations from the same code block.

Author: Vipul S. Chawathe

docs/parallel/concrt/codesnippet/CPP/walkthrough-creating-a-dataflow-agent_4.cpp

@@ -39,9 +39,9 @@
             // Set the sentinel event.
             received_sentinel.set();
          },
-         [](int value) { 
+         [](int value) -> bool { 
             return value == 0; 
          });
 
       // Connects the _source message buffer to the rest of the network.
-      unbounded_buffer<int> connector;
+      unbounded_buffer<int> connector;