Fix erroneous end tags in spec (#1497)

index.bs

@@ -243,7 +243,7 @@ Add "and [=navigate/navigationSourceEligible=] set to
 
 > [=Navigate=] <var ignore=''>targetNavigable</var>...
 
-<h3 id="monkeypatch-window-open">Window open steps</h4>
+<h3 id="monkeypatch-window-open">Window open steps</h3>
 
 Modify the [=tokenize the features argument=] as follows:
 
@@ -484,7 +484,7 @@ To <dfn>set Attribution Reporting headers</dfn> given a [=request=] |request|:
 1. [=header list/Set a structured field value=] given
     ("<code>[=Attribution-Reporting-Support=]</code>", |supportDict|) in |headers|.
 
-<h3 id="monkeypatch-fetch">Fetch monkeypatches</h4>
+<h3 id="monkeypatch-fetch">Fetch monkeypatches</h3>
 
 Modify [=fetch=] as follows:
 
@@ -517,7 +517,7 @@ add the step
     |request|'s [=request/Attribution Reporting eligibility=] to the result of
     [=get an eligibility from AttributionReportingRequestOptions=] with it.
 
-<h3 id="monkeypatch-xmlhttprequest">XMLHttpRequest monkeypatches</h4>
+<h3 id="monkeypatch-xmlhttprequest">XMLHttpRequest monkeypatches</h3>
 
 An {{XMLHttpRequest}} object has an associated
 <dfn id="xmlhttprequest-eligibility">Attribution Reporting eligibility</dfn> (an