Use SPDX identifier in AppStream metadata

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-metadata_license

appstream/vim-command-t.metainfo.xml

@@ -7,6 +7,6 @@
 <summary>Provides an extremely fast, intuitive mechanism for opening files with a minimal number of keystrokes</summary>
 <url type="homepage">https://github.com/wincent/command-t</url>
 <metadata_license>CC0-1.0</metadata_license>
-<project_license>BSD</project_license>
+<project_license>BSD-2-Clause</project_license>
 <updatecontact>[email protected]</updatecontact>
 </component>