It is, however, useful to tell the operating system to load it whenever you boot. This requires edits to two files. The first one is currently empty, so just echo over the required line:
# echo geom_mirror_load="YES" > /boot/loader.conf
I would suggest it best to be in the habit of using '>>' rather than '>' except in situations one expressly desires a new file to be started from scratch. Its too likely something else is already in /boot/loader.conf to use '>'.