Add missing value to ffsim (#704)

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### Summary

This broke the website generation. I'm not sure why the PR didn't fail
CI as events created by actions cannot trigger other actions. More
information here:
https://github.com/peter-evans/create-pull-request/blob/main/docs/concepts-guidelines.md#triggering-further-workflow-runs.

The workaround that seems most appropriate is to add better branch
protections, and close/reopen PRs to trigger the checks. I've addressed
this by explicitly requiring the **tests (3.9)** check to pass before
merging.

ecosystem/resources/members/ffsim.toml

@@ -7,3 +7,4 @@ alternatives = "_No response_"
 affiliations = "IBM"
 labels = [ "Chemistry", "Circuit simulator", "IBM maintained", "Rust",]
 website = "https://qiskit-community.github.io/ffsim/"
+group = "applications"