Rework readme links (#182)

As suggested during the discussions, this PR renames the link to the specification, and removes the link to the GitHub issues.

README.md

@@ -13,12 +13,9 @@ OpenPBR is an open standard hosted by the [Academy Software Foundation](https://
 
 ### Resources
 
-* **[White paper](https://academysoftwarefoundation.github.io/OpenPBR/)**
+* **[Specification](https://academysoftwarefoundation.github.io/OpenPBR/)**
 * **[Reference implementation](reference/open_pbr_surface.mtlx)** – written in [MaterialX](https://materialx.org/)
 
-### Discussion
-* **[GitHub issues](https://github.com/AcademySoftwareFoundation/OpenPBR/issues)**
-
 <br/>
 
 [![License: CC BY-SA 4.0](https://img.shields.io/badge/License-Apache%202.0-informational.svg)](LICENSE)