Fix typo in compose.txt

Reported by James Wood.

The line

    NG     ŊF      1

looks indeed like a typo.

From /usr/share/X11/locale/en_US.UTF-8/Compose:

<Multi_key> <N> <G>              	: "Ŋ"   U014A # LATIN CAPITAL LETTER ENG
<Multi_key> <n> <g>              	: "ŋ"   U014B # LATIN SMALL LETTER ENG
Release 1.3.6