Update Vimdoc based on README

Author: github-actions[bot]

doc/dirvish-do.txt

@@ -40,7 +40,7 @@ Table of Contents                               *dirvish-do-table-of-contents*
 - Nvim 0.8 or later
 - dirvish.vim <https://github.com/justinmk/vim-dirvish>
 - Optional : if you use move-to-trash feature
-    - You need to install Python3 provider for Neovim. See |provider-python| for more information
+    - You need to install Python3 provider for Neovim. You can check if `:echo has('python3')` returns 1 to knwo whether Python3 provider is installed. See |provider-python| for more information
     - You also need to install Python library send2trash <https://pypi.org/project/Send2Trash/>