docs: submitting-patches: adjust Fixes definition slightly

Every now and then people send stylistic patches and use Fixes
purely to refer to a commit which added the ugly or unnecessary
code. Reword the docs about Fixes.

It should hopefully be enough to lead with the word "bug"
rather than "issue". We can add more verbiage later, tho, let's
try the word swap first. I always feel like the more words the
smaller the chance someone will actually read the docs.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250904144533.2146576-1-kuba@kernel.org>