Update CI badge for Coq opam package

Author: JasonGross

README.md

@@ -1,6 +1,6 @@
 Rewriter
 =====
-[![CI (Coq)](https://github.com/mit-plv/rewriter/actions/workflows/coq.yml/badge.svg?branch=master)](https://github.com/mit-plv/rewriter/actions/workflows/coq.yml?query=branch%3Amaster)
+[![CI (Coq, opam)](https://github.com/mit-plv/rewriter/actions/workflows/coq-opam-package.yml/badge.svg?branch=master)](https://github.com/mit-plv/rewriter/actions/workflows/coq-opam-package.yml?query=branch%3Amaster)
 [![CI (Coq, dev, Docker)](https://github.com/mit-plv/rewriter/actions/workflows/coq-docker.yml/badge.svg?branch=master)](https://github.com/mit-plv/rewriter/actions/workflows/coq-docker.yml?query=branch%3Amaster)
 [![CI (Coq, Alpine)](https://github.com/mit-plv/rewriter/actions/workflows/coq-alpine.yml/badge.svg?branch=master)](https://github.com/mit-plv/rewriter/actions/workflows/coq-alpine.yml?query=branch%3Amaster)