I don't think there is any difference.  I tried both and worked fine
SQL>
SQL> create procedure testing is
  2 
  3  begin
  4 
  5  dbms_output.put_line('Testing for IS Syntax');
  6 
  7  end;
  8 
  9  /

Procedure created.

SQL> drop  procedure testing ;

Procedure dropped.

SQL> create procedure testing as
  2 
  3  begin
  4 
  5  dbms_output.put_line('Testing for AS Syntax');
  6 
  7  end;
  8 
  9  /

Procedure created.