nvim: Change default foldlevel to 3

Previously, the foldlevel of 2 would still display second level markdown
headlines (`##`) but fold `###`.

However, most of my text analyses are taking place within 3rd level
headlines nowadays, and if vim-pandoc is allowed to have folds enabled,
it will always fold everything beyond the foldlevel *each* time you
enter a buffer of pandoc filetype.

So, simply up the foldlevel one.

nvim/.config/nvim/after/plugin/prose.vim

@@ -7,7 +7,7 @@ function! s:Prose()
   call textobj#sentence#init()
   " hide the markdown cruft
   setlocal conceallevel=2
-  setlocal foldlevel=2
+  setlocal foldlevel=3
 endfunction
 
 " invoke it manually by writing :Prose