docs: submitting-patches: Trivial - fix grammatical error

"it is a used" does not make sense.  Should be "it is used".

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201216134654.271508-1-lee.jones@linaro.org


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