Use SPDX identifier

Author: miguelsousa

package.json

@@ -17,7 +17,7 @@
   "bugs": {
     "url": "https://github.com/adobe-fonts/source-serif-pro/issues"
   },
-  "license": "SIL Open Font License 1.1",
+  "license": "OFL-1.1",
   "author": {
     "name": "Frank Grießhammer"
   },