docs: submitting-patches: Fix crossref to 'The canonical patch format'

The reference to `explicit_in_reply_to` is pointless as when the
reference was added in the form of "#15" [1], Section 15) was "The
canonical patch format".
The reference of "#15" had not been properly updated in a couple of
reorganizations during the plain-text SubmittingPatches era.

Fix it by using `the_canonical_patch_format`.

[1]: 2ae19aca ("Documentation: Add "how to write a good patch summary" to SubmittingPatches")

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Fixes: 5903019b ("Documentation/SubmittingPatches: convert it to ReST markup")
Fixes: 9b2c7677 ("Documentation/SubmittingPatches: enrich the Sphinx output")
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: stable@vger.kernel.org # v4.9+
Link: https://lore.kernel.org/r/64e105a5-50be-23f2-6cae-903a2ea98e18@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>