Use right notation for default allowlist value. (#244)

w3c/webappsec-permissions-policy#123 clarified the notation and types used
by allowlists and default allowlists.

Default allowlists are not allowlists themselves, so we need to use `"self"`
rather than `["self"]`.

Fixes #154

index.html

@@ -281,7 +281,7 @@ <h3>
   <p>
     The Compute Pressure API defines a [=policy-controlled feature=]
     identified by the token "compute-pressure".
-    Its [=policy-controlled feature/default allowlist=] is `["self"]`.
+    Its [=policy-controlled feature/default allowlist=] is `'self'`.
   </p>
   <p>
     Workers (dedicated and shared) adhere to the permission policy set by their
@@ -307,7 +307,7 @@ <h3>
   </aside>
   <aside class="note">
     <p>
-      The [=policy-controlled feature/default allowlist=] of `["self"]` allows usage in
+      The [=policy-controlled feature/default allowlist=] of `'self'` allows usage in
       same-origin nested frames but prevents third-party content from using
       the feature.
     </p>