Correct reference

Author: mrotteveel

src/docs/asciidoc/en/firebirddocs/nullguide/firebird-null-guide.adoc

@@ -13,7 +13,7 @@ Paul Vinkenoog
 :imagesdir: ../../images
 
 Copyright (C) 2005-2025 Firebird Project and all contributing authors, under the https://firebirdsql.org/docs/html/en/licenses/pdl/public-documentation-license.html[Public Documentation License Version 1.0].
-Please refer to the <<gfix-license>> in the Appendix.
+Please refer to the <<nullguide-license>> in the Appendix.
 
 ////
 NOTE: Some sections have a secondary id like [[d0e33986]].