fix: correct Vim fzf reducer function in documentation

The reducer should return the whole line as-is since each line
is already a complete comment command.

docs/src/features/commands.md

@@ -211,7 +211,7 @@ if executable('presenterm') && executable('fzf')
   inoremap <expr> <c-k> fzf#vim#complete(fzf#wrap({
         \ 'source':  'presenterm --list-comment-commands',
         \ 'options': '--header "Comment Command Selection" --no-hscroll',
-        \ 'reducer': { lines -> split(lines[0])[0] } }))
+        \ 'reducer': { lines -> lines[0] } }))
 endif
 ```