Update README.md

Update the link to the vimhelp page.

Author: k-takata

README.md

@@ -2,7 +2,7 @@
 
 This project provides an HTML version of the Vim help pages at <https://vim-jp.org/vimdoc-en/>.
 
-There is already another similar site: <http://vimhelp.appspot.com/>.
+There is already another similar site: <https://vimhelp.org/>.
 The main purpose of the vimdoc-en project is to provide a handy way to see both Japanese and English versions of the help pages.
 
 The help pages are automatically updated everyday (if there are any changes).