You could modify formula field so that it puts a "0" in front of weeks < 10. Then the ordering should work.
Add 1 more formula field that can calculate the previous field adding the following:
IF( Week_number_Trial_Account__c < 10, 'Week ' + '0' + TEXT(Week_number_Trial_Account__c) , 'Week ' + TEXT(Week_number_Trial_Account__c))