bug#5688: mention one needs setq-default to change the default major-mode

[Chong Yidong]

> In (info "(emacs) Choosing Modes")
> After
>       When you visit a file that does not specify a major mode to use, or
>    when you create a new buffer with `C-x b', the default value of the
>    variable `major-mode' specifies which major mode to use.  Normally its
>    value is the symbol `fundamental-mode', which specifies Fundamental
>    mode.
> Please add
>    To change it to e.g., text-mode, in your .emacs use
>       (setq-default major-mode 'text-mode)
>
> Else beginners (and enders) will just try setq.

Fixed, thanks.