use UK spelling

Author: Xronophobe

chapter_13_database_layer_validation.asciidoc

@@ -621,7 +621,7 @@ We should also remind ourselves not to forget to remove this early return:
 
 And now, we can focus on making our code a little neater.
 
-TIP: When working on a new feature, it's common to realize partway through that
+TIP: When working on a new feature, it's common to realise partway through that
     a refactor of the application is needed. Adding an early return to the FT
     you're currently working on allows you to perform this refactor against
     passing FTs, even while the feature is still in progress.