docs: fix typo in fw.md

In docs/firmware/fw.md, there's a configuration parameter called
FW_TEXT_ADDR, which actually should be FW_TEXT_START, so fix it.

Signed-off-by: Shengyu Qu <wiagn233@outlook.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>