Valid license expression

Author: janodvarko

package.json

@@ -19,7 +19,7 @@
   "engines": {
     "firefox": ">=36.0"
   },
-  "license": "BSD License",
+  "license": "BSD-3-Clause",
   "icon": "chrome://pixelperfect/skin/logo_32x32.png",
   "homepage": "https://github.com/firebug/pixel-perfect/wiki",
   "forum": "https://groups.google.com/forum/#!forum/firebug",