update most recent supported core USD commit in build.md

This identifies core USD commit 5d1771d (post 20.08 release) as the most recent
supported version of USD:

PixarAnimationStudios/OpenUSD@5d1771d

doc/build.md

@@ -31,7 +31,7 @@ See Pixar's official github page for instructions on how to build USD: https://g
 
 |               |      ![](images/pxr.png)          |        
 |:------------: |:---------------:                  |
-|  CommitID/Tags | release: [v19.11](https://github.com/PixarAnimationStudios/USD/releases/tag/v19.11) or [v20.02](https://github.com/PixarAnimationStudios/USD/releases/tag/v20.02) or [v20.05](https://github.com/PixarAnimationStudios/USD/releases/tag/v20.05) or [v20.08](https://github.com/PixarAnimationStudios/USD/releases/tag/v20.08) <br> dev: [8a13d20](https://github.com/PixarAnimationStudios/USD/commit/8a13d20062d14d486b7e130d252c4652aab0263e) |
+|  CommitID/Tags | release: [v19.11](https://github.com/PixarAnimationStudios/USD/releases/tag/v19.11) or [v20.02](https://github.com/PixarAnimationStudios/USD/releases/tag/v20.02) or [v20.05](https://github.com/PixarAnimationStudios/USD/releases/tag/v20.05) or [v20.08](https://github.com/PixarAnimationStudios/USD/releases/tag/v20.08) <br> dev: [5d1771d](https://github.com/PixarAnimationStudios/USD/commit/5d1771d23b935740bcd5bfb985a92a6652e070c0) |
 
 For additional information on building Pixar USD, see the ***Additional Build Instruction*** section below.