Documentation/process: Co-developed-by instead of Co-Developed-by

Up to now, all commit messages have used the "d" in lower case.
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/5.Posting.rst

@@ -213,7 +213,7 @@ The tags in common use are:
    which can be found in Documentation/process/submitting-patches.rst.  Code without a
    proper signoff cannot be merged into the mainline.
 
- - Co-Developed-by: states that the patch was also created by another developer
+ - Co-developed-by: states that the patch was also created by another developer
    along with the original author.  This is useful at times when multiple
    people work on a single patch.  Note, this person also needs to have a
    Signed-off-by: line in the patch as well.