Avoid copying extraneous files in opam-devel example

opam-devel.opam

@@ -28,7 +28,7 @@ depends: [
 ]
 post-messages: [
 "The development version of opam has been successfully compiled into %{lib}%/%{name}%. You should not run it from there, please install the binaries to your PATH, e.g. with
-    sudo cp %{lib}%/%{name}%/* /usr/local/bin
+    sudo cp %{lib}%/%{name}%/opam /usr/local/bin
 
 If you just want to give it a try without altering your current installation, you could use instead:
     alias opam2=\"OPAMROOT=~/.opam2 %{lib}%/%{name}%/opam\""