[book] fix the cycle fallback section to match current code

[carljm]

Update the book section on cycle fallbacks to match what actually worked for me with the current Salsa code in main branch.

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[nikomatsakis]

Looks great, left a few nits. Is there a test case or example we can point people at?

[carljm]

Is there a test case or example we can point people at?

Good call, I'll add a link to

salsa/tests/cycles.rs

Lines 132 to 141 in cd339fc

	#[salsa::tracked(recovery_fn=recover_a)]
	fn cycle_a(db: &dyn Db, abc: ABC) -> Result<(), Error> {
	abc.a(db).invoke(db, abc)
	}
	fn recover_a(db: &dyn Db, cycle: &salsa::Cycle, abc: ABC) -> Result<(), Error> {
	Err(Error {
	cycle: cycle.participant_keys().map(|k| format!("{k:?}")).collect(),
	})
	}