docs/release-calendar: restore the missing 18.1 column

Earlier commit removed the column, instead of adjusting the height.

Cc: Dylan Baker <dylan@pnwbakers.com>
Fixes: 0d4f338a116 ("docs: Update release-notes and calendar")
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
1 file changed