doc: sending_patches.rst: s/Superseeded/Superseded

This is a common typo listed in scripts/spelling.txt. Fix it to match the
patchwork status, which is superseded.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>