Take out extra colon for "Set spam probation" (Bug #1318290)

There are two colons, but only one is needed on
http://mahara/admin/users/bulk.php. To keep it
consistent with other lang strings on the page,
the colon was taken out from the lang string.

Change-Id: Ieb3fed7adc3bc5e3bd40b21d339eba94322849b5
Signed-off-by: Kristina D.C. Hoeppner <kristina@catalyst.net.nz>