Switch to (hopefully more) correct SPDX license identifier for project

The license for the project is based on a BSD-3-Clause license, but has
additional terms about potential patent infringements.

Signed-off-by: David Runge <dvzrv@archlinux.org>

PKGBUILD

@@ -6,7 +6,7 @@ pkgrel=1
 pkgdesc="PMIx Reference RunTime Environment"
 arch=(x86_64)
 url="https://github.com/openpmix/prrte"
-license=(BSD)
+license=(BSD-3-Clause)
 depends=(
   glibc
   hwloc