Update node server examples to consistently bind to 50051

src/node/examples/math_server.js

@@ -119,7 +119,7 @@ var server = new Server({
 });
 
 if (require.main === module) {
-  server.bind('0.0.0.0:7070');
+  server.bind('0.0.0.0:50051');
   server.listen();
 }
 

src/node/examples/stock_server.js

@@ -83,7 +83,7 @@ var stockServer = new StockServer({
 });
 
 if (require.main === module) {
-  stockServer.bind('0.0.0.0:8080');
+  stockServer.bind('0.0.0.0:50051');
   stockServer.listen();
 }