Apex Triggers - Trailhead

The code itself doesn't give any errors, but anytime I run it Trailhead gives me this message:

"Challenge not yet complete... here's what's wrong: Executing the trigger did not work as expected. "

Here are the instructions:

For this challenge, you need to create a trigger that, before insert or update, checks for a checkbox. If the checkbox field is true, it sets the Shipping Postal Code (whose API name is ShippingPostalCode) to be the same as the Billing Postal Code (BillingPostalCode).

• The Apex trigger must be called AccountAddressTrigger.
• The Account object will need a new custom checkbox that should have the Field Label 'Match Billing Address' and Field Name of Match_Billing_Address. The resulting API Name should be Match_Billing_Address__c.
• With AccountAddressTrigger active, if an Account has a Billing Postal Code and Match_Billing_Address__c is true, the record should have the Shipping Postal Code set to match on insert or update.

My code:

trigger AccountAddressTrigger on Account (before insert,before update) {

    for(Account a : [SELECT Id FROM Account WHERE  Match_Billing_Address__c = TRUE AND BillingPostalCode != NULL])
    {
        a.ShippingPostalCode = a.BillingPostalCode;
        update a;        
    }//end for     
}

Your Trigger is like this way.

trigger AccountAddressTrigger on Account (before insert,before update) {

    //Iterate all accounts that is updated or inserted.
    for(Account acc :Trigger.New){
        //if match is true set values.
        if(acc.Match_Billing_Address__c){
            acc.ShippingPostalCode = acc.BillingPostalCode;
        }
    }
}