Improve CUR::FileSystem example

Fixes #2766

Author: lizmat

doc/Type/CompUnit/Repository/FileSystem.pod6

@@ -27,8 +27,12 @@ C<ver>, and C<api>.
 
     # assuming one is cloned into the zef git repository...
     my $repo = CompUnit::Repository::FileSystem.new(prefix => $*CWD);
-    my $dist = $repo.candidates("Zef").head;
-    say "Zef version: " ~ $dist.meta<version>;
+    with $repo.candidates("Zef").head -> $dist {
+        say "Zef version: " ~ $dist.meta<version>;
+    }
+    else {
+        say "No candidates for 'Zef' found";
+    }
 
 =head2 method files