Only invoke last change substitution in help files

Previously the `autocmd` to replace the date on the first line if `Last
change: ` was present resulted in the cursor being moved in non `help`
filetypes due to the substitution failing. Now the substitution is only
attempted if the current file is a `help` file.

plugin/autocmds.vim

@@ -9,7 +9,7 @@ augroup benieAugroup
   au FileType * call matchadd('Todo', '^\(<<<<<<<\||||||||\|=======\|>>>>>>>\)\s\ze.*$')
 
   " Update `Last change: <date>` on write then jump back previous position
-  au BufWritePost *.txt 1s/Last change: \zs.*$/\=strftime('%Y %b %d')/e|norm!``
+  au BufWritePost *.txt silent! call do#last_change()
 
   " Read template into buffer then send line 1 to the black hold register
   au BufNewFile todo.md read ~/.vim/templates/skeleton.todo.md | 1delete _