Fix section level

[sthibaul]

texinfo would otherwise report

./fdl.texi:404: raising the section level of @appendixsubsec which is too low

[aaronchantrill]

This fixed the following error for me:

$ make all
...
fdl.texi:404: raising the section level of @appendixsubsec which is too low
make[1]: *** [Makefile:581: speechd-up.info] Error 1
make[1]: Leaving directory '~/Projects/speechd-up'
make: *** [Makefile:394: all] Error 2