Traits: write the rules' itemlist with a brace body

@itemlist[...] gave the items as arguments, which samizdat reads as
bjodat, and the comma in the prose stopped it.

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Co-Authored-By: eca-agent <git@eca.dev>

_01.Language-basics.sz

@@ -2169,10 +2169,10 @@ An implementation may be written for a generic type at particular type arguments
 
 The compiler picks the one whose arguments match. Two rules keep that choice unambiguous:
 
-@itemlist[
+@itemlist{
   @item{Two implementations may not overlap. With @code{(List int)} already there, @code{(impl (Describe (List %a)) ...)} is refused, since a @code{(List int)} would then have two.}
   @item{The type has to be known where the call is. @code{(describe (list))} is an error, because an empty list could be either, and so is a call at @code{(List %a)} in a generic function: neither implementation is written for every element type. Give such a value an annotation, or write one implementation for @code{(List %a)} with a @code{where}, as in the next section.}
-]
+}
 
 @subsubsection{Conditional and blanket implementations}