Fix :VimwikiNextTask

Use the correct function for accessing 'rxListItemWithoutCB'.
`ac4d0a1d46` refactored access to vars and it appears this is one place
where `get_syntaxlocal` should have been renamed to `get_wikilocal` but
was accidentally skipped.

test/list_todo.vader

@@ -199,6 +199,23 @@ Expect (4 items toogled):
   * [X] Chap2
   End
 
+################################################################################
+# Todo list with text above
+
+Given vimwiki (TODO list):
+  Some other text
+
+  - [ ] Todo Item
+
+Execute (:VimwikiNextTask):
+  :execute "VimwikiNextTask" | execute 'normal yyp'
+
+Expect (Introduce new todo item):
+  Some other text
+
+  - [ ] Todo Item
+  - [ ] Todo Item
+
 ################################################################################
 # Numbered Todo list