Fixes broken SPEC link in README

[auyer]

SPEC links yielded 404 because the original documentation changed. It was moved into a sub folder.

~~Both links yielded 404 because the original documentation changed. The SPEC was moved into a sub folder, but the schema one might not be directly equivalent. In the second case, I've selected the part that best represents the intentions of this section~~

Update: I've removed part (1 out of 2 links) of the contribution because the PR #63 is already fixing it, and it should be merged.

Description of change

Two URLs in the README.md were changed because the links were broken

Manual QA steps

• click the new URLs
• check if the doc is the one intended

Risks

• none

Rollback steps

• revert this branch