cal: fix multi-byte comment, explain in better way

cal: fix multi-byte comment, explain in better way

On Sun, Dec 17, 2017 at 07:47:49PM -0500, J William Piggott wrote
> * it contains multi-byte characters (which is what drew me to reading it).
> * it truncates one very important piece of the formula: ". . . (mod 7)."
> * it explains the values for 'e', but there is no 'e' in the code.
> * it doesn't include a row resolving 'e' in the table, so it is not
>    obvious what it relates to in the code.
> * without citing it as an external reference, the comment language is confusing.

Co-Author: J William Piggott <elseifthen@gmx.com>
Signed-off-by: Karel Zak <kzak@redhat.com>