Add intents parameter to basic example

docs/intro.rst

@@ -99,6 +99,8 @@ A quick example to showcase how events work:
 
 .. code-block:: python3
 
+    # This example requires the 'message_content' intent.
+
     import discord
 
     class MyClient(discord.Client):
@@ -108,6 +110,9 @@ A quick example to showcase how events work:
         async def on_message(self, message):
             print(f'Message from {message.author}: {message.content}')
 
-    client = MyClient()
+    intents = discord.Intents.default()
+    intents.message_content = True
+
+    client = MyClient(intents=intents)
     client.run('my token goes here')