Build instructions are out of date

[zoep]

Installing CertiCoq dependencies from INSTALL.md, fails. In particular pinning the dependencies with opam pin -n -y submodules/metacoq and executing opam install coq-certicoq --deps-only fails with:

[ERROR] Package conflict!
  * Missing dependency:
    - coq-metacoq-erasure-plugin = 1.3.2+8.19
    not available because the package is pinned to version 8.19.dev

The above proceeds after unpinning the submodules with opam unpin -n -y submodules/metacoq.