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

[Michael Snyder]

Eli Zaretskii wrote:
>> 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.

I think the reason it doesn't work now is that it's typed as a void pointer.

(gdb) pt $esp
type = void *

Probably it used to be typed as a char * or something.