docs: Fix a typo in mypy documentation.
docs: Fix a typo in mypy documentation.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip