Fix License: field

The LICENSE text apears to be "3-clause BSD", and this BSD variant is
under `BSD`:

    https://fedoraproject.org/wiki/Licensing:BSD?rd=Licensing/BSD#3ClauseBSD

ydiff.spec.in

@@ -3,7 +3,7 @@ Version:    __APP_VERSION__
 Release:    __APP_RELEASE__%{?dist}
 Summary:    View colored, incremental diff
 URL:        https://github.com/ymattw/ydiff
-License:    BSD-3
+License:    BSD
 Source0:    __APP_URLBASE__/archive/%{version}/%{name}-%{version}.tar.gz
 BuildRequires: python2-devel
 BuildArch: noarch