Getting error for Create SOQL Queries in Apex Classes

While doing the challenge this error is coming, I am not sure what to do. Can someone please help ?

This is the error :

We can’t find the correct results format. Make sure that your concatenation puts each list item in this format: 'Account Name' : 'Annual Revenue'.

The code :

public class AccountUtility 
{
	public static void viewAnnualRevenue()
    {
     	List<Account> accountsList=[SELECT Name, AnnualRevenue from Account];
        for(Account act: accountsList)
        {
            String accRev=act.Name+' : '+act.AnnualRevenue;
            System.debug(accRev);
        }
    }
}