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.

autoload/do.vim

@@ -78,3 +78,11 @@ function! do#debug(...)
   endfor
   exec l:command
 endfunction
+
+function! do#last_change()
+  if &filetype ==# 'help'
+    " vint: next-line -ProhibitCommandRelyOnUser -ProhibitCommandWithUnintendedSideEffect
+    1s/Last change: \zs.*$/\=strftime('%Y %b %d')/e
+    norm!``
+  endif
+endfunction