Fix typo in reference to variable |effectiveDomain|

Fixes this Bikeshed lint:

```
LINE ~3100: The var 'effective domain' (in global scope) is only used once.
If this is not a typo, please add an ignore='' attribute to the <var>.
```

index.bs

@@ -3098,7 +3098,7 @@ The [$Asynchronous RP ID validation algorithm$] lets [=signal methods=] validate
 and returns a promise that rejects if the validation fails. The steps are:
 
 1. Let |effectiveDomain| be the [=relevant settings object=]'s [=environment
-    settings object/origin=]'s [=effective domain=]. If |effective domain| is
+    settings object/origin=]'s [=effective domain=]. If |effectiveDomain| is
     not a [=valid domain=], then return [=a promise rejected with=]
     "{{SecurityError}}" {{DOMException}}.
 1. If |rpId| [=is a registrable domain suffix of or is equal to=]