LICENSE: Clarify which BSD license is being used

Sister MR to iris/iris!472.

LICENSE: Clarify which BSD license is being used
d6ab6561 · Paolo G. Giarrusso · c8129a37 · d6ab6561
Verified Commit d6ab6561 authored 4 years ago by Paolo G. Giarrusso
--- a/LICENSE
+++ b/LICENSE
-All files in this development are distributed under the terms of the BSD
+All files in this development are distributed under the terms of the 3-clause
-license, included below.
+BSD license (https://opensource.org/licenses/BSD-3-Clause), included below.
 Copyright: std++ developers and contributors