Fix links and update preprint status

Author: mkeeter

README.md

@@ -1,9 +1,9 @@
 # `mpr`
 This is the **reference implementation** for
 [Massively Parallel Rendering of Complex Closed-Form Implicit Surfaces](https://mattkeeter.com/research/mpr),
-a technical paper which will be presented (virtually) at [SIGGRAPH 2020](s2020.siggraph.org/).
+a technical paper which will be presented (virtually) at [SIGGRAPH 2020](https://s2020.siggraph.org/).
 
-[Paper PDF](https://mattkeeter.com/research/mpr/keeter_mpr20.pdf) (preprint, 4.6 MB, no article number)
+[Paper PDF](https://mattkeeter.com/research/mpr/keeter_mpr20.pdf) (preprint, 4.6 MB)
 
 ## Citation
 ```