Bug 6525: hold filled emails not going out

Bug 6525: hold filled emails not going out

When the message name was updated from "Hold Filled" to "Hold_Filled" for T::T compatibility,
the line in _koha_notify_reserve that looked up the letter code by the message name no longer
matched, so none of the messages were sent.  This patch just replaces the space with an underscore
to get the names aligned properly.

Signed-off-by: Ian Walls <ian.walls@bywatersolutions.com>
Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>