[PATCH] gdb: Add missing single quotation mark in README

[Eli Zaretskii]

> From: Tiezhu Yang <yangtiezhu@loongson.cn>
> Date: Thu,  9 May 2024 16:35:10 +0800
> 
> Maybe forgot to add a single quotation mark for --with-system-zlib
> in commit 7f0bd4202c23 ("Update gdb's configure instructions"), just
> add it.

Thanks, fixed.