Fix opam file

Bad opam file format prevents dependencies from being installed.

opam

@@ -19,7 +19,7 @@ install: [make "install"]
 remove: ["ocamlfind" "remove" "oclaunch"]
 depends: [
   "ocamlbuild"
-  "atdgen" {>= "1.9.1"} { build }
+  "atdgen" { build & >= "1.9.1" }
   "yojson"
   "base-threads"
   "core" {>= "112.35.00"}