Use `mv -T` to move rbenv rubies to asdf rubies

This allows `~/.rbenv/versions` to rename to `~/.asdf/installs/ruby` instead of ending up with `~/.asdf/installs/ruby/versions`.

README.md

@@ -63,7 +63,7 @@ directory:
 #### rbenv
 
     mkdir ~/.asdf/installs/
-    mv ~/.rbenv/versions ~/.asdf/installs/ruby/
+    mv -T ~/.rbenv/versions ~/.asdf/installs/ruby/
 
 #### chruby