Add missing title for but in list construction

Author: book

doc/Language/traps.pod6

@@ -561,6 +561,8 @@ to a power, use parentheses:
     say (-1)²;   # OUTPUT: «1␤»
     say (-1)**2; # OUTPUT: «1␤»
 
+=head2 C<but> in List Construction
+
 The operator infix:<but> is narrower than the list constructor. When providing
 a list of roles to mix in, always use parentheses.