adding colon

docs/Using-Tensorboard.md

 
 4. Open a browser window and navigate to [localhost:6006](http://localhost:6006).
 
-**Note** The default port tensorboard uses is 6006. If there is an existing session
+**Note:** The default port tensorboard uses is 6006. If there is an existing session
 running on port 6006 a new session can be launched on an open port using the --port 
 option.