Shorten max page size in [p]slash list to account for box size

[AAA3A-AAA3A]

Description of the changes

Hello,

If a bot has a lot of slash commands, as the page size only takes into account the formatting with the py language (shorted_by=8), the pages can be more than 2000 characters long and the sending of the message fails. This PR fixes this problem.

Thanks in advance,
AAA3A

Traceback (most recent call last):
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\ext\commands\core.py", line 229, in wrapped
    ret = await coro(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{USERPROFILE}\Documents\GitHub\Red-DiscordBot\redbot\core\core_commands.py", line 2334, in slash_list
    await menu(ctx, pages)
  File "{USERPROFILE}\Documents\GitHub\Red-DiscordBot\redbot\core\utils\menus.py", line 141, in menu
    await view.start(ctx)
  File "{USERPROFILE}\Documents\GitHub\Red-DiscordBot\redbot\core\utils\views.py", line 239, in start
    self.message = await ctx.send(**kwargs, ephemeral=ephemeral)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{USERPROFILE}\Documents\GitHub\Red-DiscordBot\redbot\core\commands\context.py", line 91, in send
    return await super().send(content=content, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\ext\commands\context.py", line 1016, in send
    return await super().send(
           ^^^^^^^^^^^^^^^^^^^
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\abc.py", line 1561, in send
    data = await state.http.send_message(channel.id, params=params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\http.py", line 744, in request
    raise HTTPException(response, data)
discord.errors.HTTPException: 400 Bad Request (error code: 50035): Invalid Form Body
In content: Must be 2000 or fewer in length.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\ext\commands\bot.py", line 1350, in invoke
    await ctx.command.invoke(ctx)
  File "{USERPROFILE}\Documents\GitHub\Red-DiscordBot\redbot\core\commands\commands.py", line 791, in invoke
    await super().invoke(ctx)
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\ext\commands\core.py", line 1642, in invoke
    await ctx.invoked_subcommand.invoke(ctx)
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\ext\commands\core.py", line 1023, in invoke
    await injected(*ctx.args, **ctx.kwargs)  # type: ignore
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "{USERPROFILE}\redenv\Lib\site-packages\discord\ext\commands\core.py", line 238, in wrapped
    raise CommandInvokeError(exc) from exc
discord.ext.commands.errors.CommandInvokeError: Command raised an exception: HTTPException: 400 Bad Request (error code: 50035): Invalid Form Body
In content: Must be 2000 or fewer in length.

Have the changes in this PR been tested?

Yes

[Jackenmen]

Looks good, thanks for your contribution!