mgr/snap_schedule: change minute duration specifier from M to m

mgr/snap_schedule: change minute duration specifier from M to m

Problem:
As per the issue tracker, the period spec specifier 'M' is not
consistent with what is used elsewhere, like the period specifiers
displayed in the 'ceph status' command output.
The 'M' period specifier is used as a 'minute' level period specifier by
the cephfs team.
The issue reporter suggests to use 'M' as a 'month' period specifier.

Solution:
Since the 'minute' level period specifer, 'M', is used internally by
the development team, it is failrly easy to swap the 'minute' ('M')
level and 'month' ('m') level period specifers to finally mean that 'm'
implies 'minute' level period and 'M' implies 'month' level period.
Also, since this is the first time that somebody has ever reported that
neither the 'M' nor the 'm' level specifiers work in production, it is a
good idea to fix them once and for all.

Fixes: https://tracker.ceph.com/issues/62494
Signed-off-by: Milind Changire <mchangir@redhat.com>
(cherry picked from commit 2f6d87d1811b745c4efcac72c03ddb42751ad634)