erasure-code: fix URL in developer notes

add missing ..

http://tracker.ceph.com/issues/8388 Fixes: #8388

Reported-by: koleosfuscus <koleosfuscus@yahoo.com>
Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1824 from dachary/wip-documentation

erasure-code: fix URL in developer notes

Reviewed-by: koleosfuscus <koleosfuscus@yahoo.com>