Update ref to license of used project in README

Google relicensed the Material Design Icons to the Apache 2.0
license for the version 3.0 release in the follow commit:
google/material-design-icons@68e015d

Author: Kasijjuf

README.md

@@ -245,4 +245,4 @@ And that's all there is to it.
 Released under the [Apache 2.0 License](https://github.com/code-mc/material-icon-lib/blob/master/license.md)
 
 * __MaterialDesignicons__ Released under the [SIL Open Font License 1.1](http://opensource.org/licenses/OFL-1.1)
-* __Google Material Design Icons__ Released under the [Attribution 4.0 International License](https://github.com/google/material-design-icons/blob/master/LICENSE)
+* __Google Material Design Icons__ Released under the [Apache 2.0 License](https://github.com/google/material-design-icons/blob/master/LICENSE)