Update composer.json

Reading composer.json of dkrnl/simplexmlreader (master)
Skipped branch master, Invalid package information: 
License "Public Domain" is not a valid SPDX license identifier, see https://spdx.org/licenses/ if you use an open license.
If the software is closed-source, you may use "proprietary" as license.

Author: dkrnl

composer.json

@@ -12,7 +12,7 @@
     "autoload": {
         "files": ["autoload.php"]
     },
-    "license": "Public Domain",
+    "license": "PDDL-1.0",
     "require": {
         "php": ">=5.3.0",
         "lib-libxml" : "*",