Change example of sorting multidimmed

Although the syntax "sort(*[1])" is technically correct, I felt it could
be confusing to naive readers with the "say @A[*;1]" syntax just two lines
before it.  Using "sort( { $_[1] } )" is less magic at a place where
there's already quite some magic introduced.

doc/Language/subscripts.pod6

@@ -355,7 +355,7 @@ multidimensional subscripts.
     say @a[*;1]; # 2nd element of each sub list
     # OUTPUT: «(2 4)␤»
     my @a = (<1 c 6>, <2 a 4>, <5 b 3>);
-    say @a.sort(*[1]); # sort by 2nd column
+    say @a.sort( { $_[1] } ); # sort by 2nd column
     # OUTPUT: «((2 a 4) (5 b 3) (1 c 6))␤»
 
 =head1 Modifying elements