Fix escape sequence in tails-gdm-failed-to-start.service (refs: #17166)

The command works, but produces this message:

    /lib/systemd/system/tails-gdm-failed-to-start.service:11: Ignoring
    unknown escape sequences: "MAX_LENGTH=254 ;       PREFIX="Error starting
    GDM with your graphics card: " ;       SUFFIX=". Please take note of
    this error and visit https://tails.boum.org/gdm for troubleshooting." ;
          MAX_VIDEO_CARD_LENGTH=$(($MAX_LENGTH - $(echo -n "$PREFIX$SUFFIX"
    | wc -c))) ;       VIDEO_CARD=$(lspci -d::0300 -nn | sed -E "s,.* VGA
    compatible controller \[0300\]: *,," | cut -c
    "1-$MAX_VIDEO_CARD_LENGTH") ;       /bin/plymouth display-message
    --text="$PREFIX$VIDEO_CARD$SUFFIX"      "

Adding a backslash fixes that and the command still works.