fix the etc for the backup program

[segasai]

Thanks to @dkirkby @schlafly (in desihub/desispec#2370 ) I was able to run ETC on an exposure, and
looking at the json allowed to me figure out why the backup program switch doesn't trigger.
The check was written as

desietc/desietc/etc.py

Line 1052 in c668ed8

if self.fassign_data['FAPRGRM'] != 'BACKUP':

FAPRGRM !='BACKUP' while it's actually is 'backup' :)

This patch fixes that.

I checked that for the exposure 252068 before the etc was returning :
"MW_transp": 0.613693618072619
while with the patch it does
"MW_transp": 1

[schlafly]

Beautiful, this looks good to me! So when we merge & install this at KP we should also merge the corresponding update on desispec, and then these tiles start working right. Thank you!