doc: sfp-phylink: Fix a broken reference

The commit in Fixes: has changed a .txt file into a .yaml file. Update the
documentation accordingly.

While at it add some `` around some file names to improve the output.

Fixes: 70991f1e

 ("dt-bindings: net: convert sff,sfp to dtschema")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/be3c7e87ca7f027703247eccfe000b8e34805094.1659247114.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>