Fix LICENSE file

I was a bit too quick in copying OCaml's; replaced references to OCaml with references to opam

LICENSE

@@ -1,22 +1,17 @@
-In the following, "the OCaml Core System" refers to all files marked
-"Copyright INRIA" in this distribution.
-
-The OCaml Core System is distributed under the terms of the
+Opam is distributed under the terms of the
 GNU Lesser General Public License (LGPL) version 2.1 (included below).
 
-As a special exception to the GNU Lesser General Public License, you
-may link, statically or dynamically, a "work that uses the OCaml Core
-System" with a publicly distributed version of the OCaml Core System
-to produce an executable file containing portions of the OCaml Core
-System, and distribute that executable file under terms of your
-choice, without any of the additional requirements listed in clause 6
-of the GNU Lesser General Public License.  By "a publicly distributed
-version of the OCaml Core System", we mean either the unmodified OCaml
-Core System as distributed by INRIA, or a modified version of the
-OCaml Core System that is distributed under the conditions defined in
-clause 2 of the GNU Lesser General Public License.  This exception
-does not however invalidate any other reasons why the executable file
-might be covered by the GNU Lesser General Public License.
+As a special exception to the GNU Lesser General Public License, you may link,
+statically or dynamically, a "work that uses opam" with a publicly distributed
+version of opam to produce an executable file containing portions of opam, and
+distribute that executable file under terms of your choice, without any of the
+additional requirements listed in clause 6 of the GNU Lesser General Public
+License. By "a publicly distributed version of opam", we mean either the
+unmodified opam as distributed by OCamlPro, or a modified version of the opam
+that is distributed under the conditions defined in clause 2 of the GNU Lesser
+General Public License. This exception does not however invalidate any other
+reasons why the executable file might be covered by the GNU Lesser General
+Public License.
 
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