Convert minority of "behaviour" spelling occurrences (3 lines) to spelling convention "behavior" (53 lines) used in the majority of this repository

src/exotic-sizes.md

@@ -157,9 +157,9 @@ because that wouldn't make sense.
 We recommend against modelling C's `void*` type with `*const Void`.
 A lot of people started doing that but quickly ran into trouble because
 Rust doesn't really have any safety guards against trying to instantiate
-empty types with unsafe code, and if you do it, it's Undefined Behaviour.
+empty types with unsafe code, and if you do it, it's Undefined Behavior.
 This was especially problematic because developers had a habit of converting
-raw pointers to references and `&Void` is *also* Undefined Behaviour to
+raw pointers to references and `&Void` is *also* Undefined Behavior to
 construct.
 
 `*const ()` (or equivalent) works reasonably well for `void*`, and can be made

src/subtyping.md

@@ -75,7 +75,7 @@ this stuff really works, and how we can mess it up.
 
 The core problem is that this rule, naively applied, will lead to *meowing Dogs*. That is,
 we can convince someone that a Dog is actually a Cat. This completely destroys the fabric
-of our static type system, making it worse than useless (and leading to Undefined Behaviour).
+of our static type system, making it worse than useless (and leading to Undefined Behavior).
 
 Here's a simple example of this happening when we apply subtyping in a completely naive
 "find and replace" way.