Use "opam var" instead "opam config var"

"opam config var" was deprecared in version 2.1 and generates an error message in the process output:

[WARNING] var was deprecated in version 2.1 of the opam CLI. Use opam var instead or set OPAMCLI environment variable to 2.0.

README.md

@@ -173,7 +173,7 @@ to your `~/.emacs`:
  'load-path
  (replace-regexp-in-string
   "\n" "/share/emacs/site-lisp"
-  (shell-command-to-string "opam config var prefix")))
+  (shell-command-to-string "opam var prefix")))
 
 ;; Automatically load utop.el
 (autoload 'utop "utop" "Toplevel for OCaml" t)