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

Documentation/process/submitting-patches.rst

@@ -446,7 +446,7 @@ patch.  This tag documents that potentially interested parties
 have been included in the discussion.
 
 Co-developed-by: states that the patch was co-created by multiple developers;
-it is a used to give attribution to co-authors (in addition to the author
+it is used to give attribution to co-authors (in addition to the author
 attributed by the From: tag) when several people work on a single patch.  Since
 Co-developed-by: denotes authorship, every Co-developed-by: must be immediately
 followed by a Signed-off-by: of the associated co-author.  Standard sign-off