'Murica behavioring.

https://github.com/discoverygarden/dgi_migrate/blob/main/src/Plugin/migrate/process/MissingBehaviorTrait.php#L62

src/Plugin/migrate/process/Subindex.php

@@ -25,7 +25,7 @@
  *     dereference, to attempt to use as the index to access.
  * - "skip_row_if_missing": Deprecated in favour of "missing_behavior". A flag
  *   to indicate if the row should be skipped when we find there's no value.
- * - "missing_behaviour": A string indicating what should happen when we fail
+ * - "missing_behavior": A string indicating what should happen when we fail
  *   to find the target value. One of:
  *   - "abort": Stop the migration, throwing an exception.
  *   - "skip_process": Stop processing the property.