Changed example cmd2 history file's extension from txt to dat since it is now binary

Author: kmvanbrunt

.gitignore

@@ -28,4 +28,4 @@ dmypy.json
 dmypy.sock
 
 # cmd2 history file used in hello_cmd2.py
-cmd2_history.txt
+cmd2_history.dat

examples/hello_cmd2.py

@@ -11,7 +11,7 @@
 
     # Set "use_ipython" to True to include the ipy command if IPython is installed, which supports advanced interactive
     # debugging of your application via introspection on self.
-    app = cmd2.Cmd(use_ipython=True, persistent_history_file='cmd2_history.txt')
+    app = cmd2.Cmd(use_ipython=True, persistent_history_file='cmd2_history.dat')
     app.locals_in_py = True     # Enable access to "self" within the py command
     app.debug = True            # Show traceback if/when an exception occurs
     sys.exit(app.cmdloop())