Update third party license notice for notification icon

License has been updated in September 2020:
6d3df9c742

Change-Id: I8f575a6cd871a9ee0c99eb4f55e3414ab982a865

graphics/materialdesignicons/README.md

@@ -1,5 +1,5 @@
 Icons in this directory have been taken from [materialdesignicons.com](https://materialdesignicons.com/),
-released under the [SIL Open Font License 1.1](https://scripts.sil.org/cms/scripts/page.php?item_id=OFL_web).
+released under the [Apache 2.0 license](https://www.apache.org/licenses/LICENSE-2.0).
 
 Listed below are the original icon names and authors:
 

vector/src/main/assets/open_source_licenses.html

@@ -396,6 +396,11 @@ SOFTWARE.
         <br/>
         Copyright (c) 2017-present, dialog LLC &lt;info@dlg.im&gt;
     </li>
+    <li>
+        <b>tortoise</b>
+        <br/>
+        Copyright 2018, Nick / materialdesignicons.com
+    </li>
 </ul>
 <pre>
 Apache License