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/6d8d30f6bda84e1b711121bb98a07a464d3f089aReviewed-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>

Documentation/process/handling-regressions.rst

@@ -432,7 +432,7 @@ or itself is a reply to that mail:
  * Mark a regression as fixed by a commit that is heading upstream or already
    landed::
 
-       #regzbot fixed-by: 1f2e3d4c5d
+       #regzbot fix: 1f2e3d4c5d
 
  * Mark a regression as a duplicate of another one already tracked by regzbot::