A small grammar fix app_commands.context_menu

This just makes it so it adheres with the rule that `an` goes 
before a word that begins with a vowel.

discord/app_commands/commands.py

@@ -1310,7 +1310,7 @@ def command(
 
 
 def context_menu(*, name: str = MISSING) -> Callable[[ContextMenuCallback], ContextMenu]:
-    """Creates a application command context menu from a regular function.
+    """Creates an application command context menu from a regular function.
 
     This function must have a signature of :class:`~discord.Interaction` as its first parameter
     and taking either a :class:`~discord.Member`, :class:`~discord.User`, or :class:`~discord.Message`,