Update comments of to_vote_instruction failure. (#3670)

Update comment, to_vote_instruction could have more than switch threshold failures.

Author: wen-coding

core/src/replay_stage.rs

@@ -2611,7 +2611,7 @@ impl ReplayStage {
                 vote_account_pubkey,
                 &authorized_voter_keypair.pubkey(),
             )
-            .expect("Switch threshold failure should not lead to voting");
+            .expect("Switch failure should not lead to voting");
 
         let mut vote_tx = Transaction::new_with_payer(&[vote_ix], Some(&node_keypair.pubkey()));