internal: fix typo in comments (#18106)

Changed "signTransactions" to "signTransaction"

internal/ethapi/api.go

@@ -339,7 +339,7 @@ func (s *PrivateAccountAPI) LockAccount(addr common.Address) bool {
 	return fetchKeystore(s.am).Lock(addr) == nil
 }
 
-// signTransactions sets defaults and signs the given transaction
+// signTransaction sets defaults and signs the given transaction
 // NOTE: the caller needs to ensure that the nonceLock is held, if applicable,
 // and release it after the transaction has been submitted to the tx pool
 func (s *PrivateAccountAPI) signTransaction(ctx context.Context, args *SendTxArgs, passwd string) (*types.Transaction, error) {