Tune down recommendation and point to bolero

[celinval]

Description of changes:

I don't think we should discourage people from looking into Kani so I'm changing the words here. I also changed the reference to point to bolero instead of proptest.

Resolved issues:

N/A

Related RFC:

Optional #ISSUE-NUMBER.

Call-outs:

Testing:

• How is this change tested?

• Is this a refactor change?

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

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