docs: use american english customize (#18365)

docs/usage/configuration-options.md

@@ -2359,7 +2359,7 @@ Avoid setting `rebaseWhen=never` and then also setting `prCreation=not-pending`
 
 By default, Renovate will detect if it has proposed an update to a project before and not propose the same one again.
 For example the Webpack 3.x case described above.
-This field lets you customise this behavior down to a per-package level.
+This field lets you customize this behavior down to a per-package level.
 For example we override it to `true` in the following cases where branch names and PR titles need to be reused:
 
 - Package groups

docs/usage/getting-started/installing-onboarding.md

@@ -94,7 +94,7 @@ The `package.json` file must be located at the root of your repository.
 This is handy if you are already using a `package.json` file anyway, e.g. when you're working on a JavaScript project.
 The configuration in your `package.json` will apply to the whole project (this includes other, nested `package.json` files).
 
-### Customised defaults
+### Customized defaults
 
 Renovate provides a `renovate.json` file, with default settings that will work for most cases.
 Sometimes Renovate detects that an override to these defaults is needed, and will add this override automatically, such as: