Fix mis-spelled vim routes file link.

In the README.md it links to

- [Open Routes File With vim-rails](vim/open-routes-file-with-vim-rails.md)

However, the md file is actually called open-routes-files. This fix
renames the vim/open-routes-files to the singular version.

vim/open-routes-file-with-vim-rails.md

@@ -0,0 +1,12 @@
+# Open Routes File With vim-rails
+
+You can quickly pop open the `config/routes.rb` file for a Rails project by
+using the following command provided by
+[`vim-rails`](https://github.com/tpope/vim-rails).
+
+```
+:Einit
+```
+
+This is a shortcut for opening initializer files the default of which is the
+routes file.