only register MaxCandidates-1 candidates initially (#1714)

cumulus/pallets/collator-selection/src/benchmarking.rs

@@ -147,7 +147,7 @@ benchmarks! {
 	// worse case is when we have all the max-candidate slots filled except one, and we fill that
 	// one.
 	register_as_candidate {
-		let c in 1 .. T::MaxCandidates::get();
+		let c in 1 .. T::MaxCandidates::get() - 1;
 
 		<CandidacyBond<T>>::put(T::Currency::minimum_balance());
 		<DesiredCandidates<T>>::put(c + 1);