Add a $*ERR.say on a simple catch block as example of how to use STDERR.

See discussion in issue #1722.

Author: fluca1978

doc/Language/exceptions.pod6

@@ -52,13 +52,13 @@ It's possible to handle exceptional circumstances by supplying a C<CATCH> block:
     die X::IO::DoesNotExist.new(:path("foo/bar"), :trying("zombie copy"));
 
     CATCH {
-        when X::IO { say "some kind of IO exception was caught!" }
+        when X::IO { $*ERR.say: "some kind of IO exception was caught!" }
     }
 
     # RESULT: «some kind of IO exception was caught!»
 
 Here, we are saying that if any exception of type C<X::IO> occurs, then the
-message C<some kind of IO exception was caught!> will be displayed.
+message C<some kind of IO exception was caught!> will be sent to I<stderr> and displayed.
 
 A X<C<CATCH>|CATCH> block uses smart matching similar to how C<given/when> smart
 matches on options, thus it's possible to catch and handle various categories of