maunal: update firmware filenaming

In Change-Id I319edb76dd74be6904395692ab1599fb8f1f9065 we had
changed the file naming of the firmware binaries from the somewhat
general fw_app*.bin format to icE1usb-fw*.bin, but the manual
was not updated accordingly.

Change-Id: I0cc2cd3f3f8f166b9409463fbd4b01e344cfe399
1 file changed