docs: handling-regressions.rst: Update regzbot command fixed-by to fix

On the reference documentation for regzbot, the fixed-by command has
been renamed to fix. Update the kernel documentation accordingly.

Link: https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md
Link: https://gitlab.com/knurd42/regzbot/-/commit/6d8d30f6bda84e1b711121bb98a07a464d3f089a


Reviewed-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: "Nícolas F. R. A. Prado" <nfraprado@collabora.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20240311-regzbot-fixes-v2-2-98c1b6ec0678@collabora.com>