Tune down recommendation and point to bolero (rust-lang#2212)

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.

Author: celinval

docs/src/application.md

@@ -6,9 +6,8 @@ You may be interested in applying Kani if you're in this situation:
 2. You've already invested heavily in testing to ensure correctness.
 3. You want to invest further, to gain a much higher degree of assurance.
 
-> If you haven't already, we recommend techniques like property testing (e.g. with [`proptest`](https://github.com/AltSysrq/proptest)) before attempting model checking.
-> These yield good results, are very cheap to apply, and are often easier to adopt and debug.
-> Kani is a next step: a tool that can be applied once cheaper tactics are no longer yielding results, or once the easier to detect issues have already been dealt with.
+> If you haven't already, we also recommend techniques like property testing and fuzzing (e.g. with [`bolero`](https://github.com/camshaft/bolero/)).
+> These yield good results, are very cheap to apply, and are often easy to adopt and debug. 
 
 In this section, we explain [how Kani compares with other tools](./tool-comparison.md)
 and suggest [where to start applying Kani in real code](./tutorial-real-code.md).