Fix missing bracket in ex3_6

Exercises/ex3_6.md

@@ -146,7 +146,7 @@ Try it out:
 >>> from tableformat import create_formatter
 >>> formatter = create_formatter('text')
 >>> with redirect_stdout(open('out.txt', 'w')) as file:
-        tableformat.print_table(portfolio, ['name','shares','price', formatter)
+        tableformat.print_table(portfolio, ['name','shares','price'], formatter)
         file.close()
 
 >>> # Inspect the file