Update PDF link

Author: sqzsq

README.md

@@ -9,5 +9,4 @@ Files:
 - [Charter adoc](https://github.com/riscv/configuration-structure/blob/master/charter.adoc): the group charter
 - [Charter pdf](https://github.com/riscv/configuration-structure/blob/master/charter.pdf): pre-built PDF version of the group charter
 - [Spec adoc](https://github.com/riscv/configuration-structure/blob/master/riscv-unified-discovery-draft.adoc): the draft specification
-- [Spec pdf](https://github.com/riscv/configuration-structure/blob/master/riscv-unified-discovery-draft.pdf): pre-built PDF version of the draft specification
-- [past_work](https://github.com/riscv/configuration-structure/tree/master/past_work) folder: the work done previously
+- [Spec pdf](https://github.com/riscv/configuration-structure/releases/download/v0.7-draft/riscv-unified-discovery-draft.pdf): latest release PDF version of the draft specification