Fix function name for folding

[delphinus]

'foldexpr' & 'foldtext' do not detect script functions

Reproduce way

Vim 8.0.1283
Nvim v0.3.1

mkdir /tmp/vim
cd /tmp/vim
git clone https://github.com/freitass/todo.txt-vim
vim vimrc

set runtimepath+=/tmp/vim/todo.txt-vim
syntax enable
filetype plugin on

vim -N -u vimrc -U NONE -i NONE
:e /path/to/todo.txt