Fix plugin.xml validation

The [PhoneGap Build plugins list](https://build.phonegap.com/plugins) won't accept a plugin without a valid `plugin.xml`. After the change, this XML file passes [W3C validation](http://validator.w3.org/check).

After this fix, could you also please upload it to the PhoneGap Build plugins list?  A search for "sqlite" turns up two different forks, but not this repo:

* https://github.com/millerjames01/Cordova-SQLitePlugin
* https://github.com/huserben/Cordova-WP-SqlitePlugin