Change headline so that it does not contain the word class and a type

This is a workaround for a problem in which htmlify.p6 can be tripped
up when it encounters what looks like a type heading in a file which
is not present in the type directory. This will lead to subtitles,
i.e. descriptions, going missing on https://docs.perl6.org/type.html.
See https://irclog.perlgeek.de/perl6/2017-09-24#i_15210015

Author: Jan-Olof Hendig

doc/Programs/01-debugging.pod6

@@ -71,7 +71,7 @@ name of the current subroutine / method to the standard error stream:
 
 This can be handy as a cheap trace function.
 
-=head2 The C<Backtrace> class
+=head2 Using backtraces
 
 The L<Backtrace> class gets the current call stack, and can return it as a
 string: