Navigation tree view examples: Plus/Minus characters are being read by screen readers

[mcking65]

In the navigation tree examples:

1. Navigation Treeview Example Using Computed Properties
2. Navigation Treeview Example Using Declared Properties

The generated content used to display plus and minus symbols is being read by screen readers. It comes from the css content properties:

• content: "\00229E \0000A0";
• content: "\00229F \0000A0";

Another method, such as that used in the accordion and breadcrumb examples, or an image is needed so the visual indicators are not read.

[jongund]

Fixed in lastest pull request

[mcking65]

Thank you @jongund, this issue is resolved by commit ba5cf02.