Fix filter example

[astrofrog]

The following should include u.percent for the transmission

f = i.add_filter()
f.name = 'F2'
f.spectral_coord = [2, 2.1, 2.2, 2.3, 2.4] * u.micron
f.transmission = [0., 50, 100, 60, 0.]
f.detector_type = 'energy'
f.alpha = 1.
f.central_spectral_coord = 2.15 * u.micron