Edited appendix_validation.asciidoc with Atlas code editor

Author: katherinetozer

appendix_validation.asciidoc

@@ -193,7 +193,7 @@ TIP: Validate as little as possible. Read only the fields you need, and don't
     systems change over time. Resist the temptation to share message
     definitions between systems: instead, make it easy to define the data you
     depend on. For more info, see Martin Fowler's article on the
-    https://martinfowler.com/bliki/TolerantReader.html[Tolerant Reader pattern].
+    https://oreil.ly/YL_La[Tolerant Reader pattern].
 
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 .Is Postel Always Right?