Fix code blocks explaining $!.

This corrects my mistakes around two blocks from commit a06f33a,
as for discussion
a06f33a#commitcomment-26642734
this commit does:
- remove the :skip-test around the try code block, since it can be tested;
- add a =for code directive with shell language around the output code
  block.

Author: fluca1978

doc/Language/5to6-perlvar.pod6

@@ -530,7 +530,7 @@ or L<exception|/type/Exception>.
 In particular, when dealing with L<exceptions|/type/Exception> the C<$!> provides information
 about the thrown exception, assuming the program has not halted:
 
-=begin code :skip-test
+=begin code
 try {
     fail "Boooh";
     CATCH {
@@ -550,10 +550,11 @@ say $!.^name ~ ' : ' ~ $!.message;
 
 and the above code produces the following output
 
-    within the catch:
-    X::AdHoc : Boooh
-    outside the catch:
-    X::AdHoc : Boooh
+=for code :lang<shell>
+within the catch:
+X::AdHoc : Boooh
+outside the catch:
+X::AdHoc : Boooh
 
 therefore, as stated before, the C<$!> variable holds the exception object.