remove format code D, use C instead

D may not yet be implemented

Author: tbrowder

doc/Language/pod.pod6

@@ -30,7 +30,7 @@ You can use whichever form is most convenient for your particular documentation
 
 Delimited blocks are bounded by C<=begin> and C<=end> markers, both of
 which are followed by a valid Perl 6 identifier, which is the
-D<typename> of the block. Typenames that are entirely lowercase (for
+C<typename> of the block. Typenames that are entirely lowercase (for
 example: C<=begin head1>) or entirely uppercase (for example: C<=begin
 SYNOPSIS>) are reserved.