[doc/tracepoint] Change *$esp@300 to *(unsigned char *)$esp@300.

[Eli Zaretskii]

> From: Hui Zhu <teawater@gmail.com>
> Date: Fri, 16 Sep 2011 15:54:54 +0800
> Cc: Eli Zaretskii <eliz@gnu.org>
> 
> 2011-09-16  Hui Zhu  <teawater@gmail.com>
> 
> 	* gdb.texinfo (Tracepoint Restrictions): Change *$esp@300
> 	to *(unsigned char *)$esp@300.

OK.