Add a pair of parentheses as delimiter

Author: Marcythm

src/macros-by-example.md

@@ -174,7 +174,7 @@ compiler knows how to expand them properly:
 2.  Each repetition in the transcriber must contain at least one metavariable to
     decide how many times to expand it. If multiple metavariables appear in the
     same repetition, they must be bound to the same number of fragments. For
-    instance, `( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( ($i,$j) ),* )` must
+    instance, `( $( $i:ident ),* ; $( $j:ident ),* ) => (( $( ($i,$j) ),* ))` must
     bind the same number of `$i` fragments as `$j` fragments. This means that
     invoking the macro with `(a, b, c; d, e, f)` is legal and expands to
     `((a,d), (b,e), (c,f))`, but `(a, b, c; d, e)` is illegal because it does