doc/markdown: fix typo in dev guide

Change-Id: I3fdc2fd9a51ea823040e80bcc61290141f88a520
Signed-off-by: Ram Raja <rraja@redhat.com>
Reviewed-on: http://review.gluster.org/5600
Reviewed-by: Luis Pabon <lpabon@redhat.com>
Tested-by: Luis Pabon <lpabon@redhat.com>