Workshop 201, Tutorial 3, Step 4.3, Apex

InvoiceUtilities.cls

public class InvoiceUtilities {

    // class method to renumber Line Items for a given Invoice number
    // returns a string that indicates success or failure
    public static String renumberLineItems(String invoiceName) {

    // create a copy of the target Invoice object and it's Line Items
    Invoice__c invoice = [Select i.Name, (Select Name From Line_Items__r ORDER BY Name) 
                            From Invoice__c i 
                           Where i.Name = :invoiceName LIMIT 1];

    // loop through each Line Item, renumbering as you go
    Integer i = 1;
    for (Line_Item__c item : invoice.Line_Items__r) {
      item.Name = String.valueOf(i);
      System.debug(item.Name);
      i++;
    }

    // update the Line Items in one transaction, rollback if any problems
    // and return error messages to the calling environment
    try {
      database.update(invoice.Line_Items__r);
    }
    catch (DmlException e) {
      return e.getMessage();
    }

    // on success, return a message to the calling program
    return 'Line items renumbered successfully.'; 
  }
}

Srilaxmi15 is correct, copy and paste the invoice number from the application; for some reason it's a different dash. In the workbook, they use U+2013, En Dash, instead of what's in the record, a simple U+002D, Hyphen Minus sign.