Fix mistake in documentation.

docs/migrating.rst

@@ -985,7 +985,7 @@ For example, to implement a :class:`~.commands.HelpCommand` in a cog, the follow
 
     class MyHelpCommand(commands.MinimalHelpCommand):
         def get_command_signature(self, command):
-            return '{0.context.clean_prefix}{1.qualified_name} {1.signature}'.format(self, command)
+            return '{0.clean_prefix}{1.qualified_name} {1.signature}'.format(self, command)
 
     class MyCog(commands.Cog):
         def __init__(self, bot):