[Doc]tracepoint: change *$esp@300 to *(unsigned char *)$esp@300

[Eli Zaretskii]

> Date: Tue, 24 Aug 2010 10:35:34 -0700
> From: Michael Snyder <msnyder@vmware.com>
> CC: Hui Zhu <teawater@gmail.com>, 
>  "gdb-patches@sourceware.org" <gdb-patches@sourceware.org>
> 
> Eli Zaretskii wrote:
> >> From: Hui Zhu <teawater@gmail.com>
> >> Date: Tue, 24 Aug 2010 11:30:03 +0800
> >> Cc: gdb-patches@sourceware.org
> >>
> >> If "unsigned char" is too big, what about *(char *)$esp@300?
> > 
> > "char" is better, but not because of the length.
> > 
> > However, what I really meant is to ask whether *$esp should indeed
> > refuse to work, or is that a bug?
> 
> I imagine it must have worked at one time...

If we-all think now that it's okay for *$esp not to work, then let's
fix the docs and be done.