[FTBS] Fixed gpl2.0.texi

The @appendixsubsec entries are substituted by @appendixsec entries.

This fixes the Debian issue in sid. The report can be found at 

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=708589

```
./gpl-2.0.texi:20: raising the section level of @appendixsubsec which is too low
```