eth: fix typo (#15252)

eth/backend.go

@@ -327,7 +327,7 @@ func (s *Ethereum) StartMining(local bool) error {
 		wallet, err := s.accountManager.Find(accounts.Account{Address: eb})
 		if wallet == nil || err != nil {
 			log.Error("Etherbase account unavailable locally", "err", err)
-			return fmt.Errorf("singer missing: %v", err)
+			return fmt.Errorf("signer missing: %v", err)
 		}
 		clique.Authorize(eb, wallet.SignHash)
 	}