docs: move releases from Fridays to Wednesdays

docs: move releases from Fridays to Wednesdays

As discussed at:
https://lists.freedesktop.org/archives/mesa-dev/2018-March/188525.html

Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Juan A. Suarez Romero <jasuarez@igalia.com>
Cc: Dylan Baker <dylan.c.baker@intel.com>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: Carl Worth <cworth@cworth.org>
Cc: Mark Janes <mark.a.janes@intel.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>