arch/xtensa: fix grammar in Kconfig help text

Spell "Don't" correctly in the XTENSA_VARIANT_CUSTOM_NAME help text.

Signed-off-by: Hu Haowen <xianfengting221@163.com>
Message-Id: <20200330045436.12645-1-xianfengting221@163.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>