[css-contain-3] Fix h2/h3 mismatch

Author: Anders Hartvoll Ruud

css-contain-3/Overview.bs

@@ -163,7 +163,7 @@ Container Queries</h2>
 	</div>
 
 <h3 id="container-type">
-Creating Query Containers: the 'container-type' property</h2>
+Creating Query Containers: the 'container-type' property</h3>
 
 	<pre class='propdef'>
 		Name: container-type
@@ -262,7 +262,7 @@ Creating Query Containers: the 'container-type' property</h2>
 
 
 <h3 id="container-name">
-Naming Query Containers: the 'container-name' property</h2>
+Naming Query Containers: the 'container-name' property</h3>
 
 	<pre class='propdef'>
 		Name: container-name
@@ -339,7 +339,7 @@ Naming Query Containers: the 'container-name' property</h2>
 
 
 <h3 id="container-shorthand">
-Creating Named Containers: the 'container' shorthand</h2>
+Creating Named Containers: the 'container' shorthand</h3>
 
 	<pre class='propdef shorthand'>
 		Name: container
@@ -368,7 +368,7 @@ Creating Named Containers: the 'container' shorthand</h2>
 
 
 <h3 id="container-rule">
-Container Queries: the ''@container'' rule</h2>
+Container Queries: the ''@container'' rule</h3>
 
 	The <dfn at-rule id="at-ruledef-container">@container</dfn> rule
 	is a [=conditional group rule=] whose condition is