[css-values-5][editorial] Align subheadings

Author: fantasai

css-values-5/Overview.bs

@@ -1255,7 +1255,7 @@ Selecting the First Supported Value: the ''first-valid()'' notation</h3>
 -->
 
 <h3 id="toggle-notation">
-Toggling Between Values: ''toggle()''</h3>
+Toggling Between Values: the ''toggle()'' notation</h3>
 
 	The <dfn>toggle()</dfn> expression allows descendant elements
 	to cycle over a list of values instead of inheriting the same value.
@@ -1388,7 +1388,7 @@ Custom Property References: the ''var()'' notation</h3>
 -->
 
 <h3 id="attr-notation">
-Attribute References: the ''attr()'' function</h3>
+Attribute References: the ''attr()'' notation</h3>
 
 <!--
 Ian's proposal:
@@ -1507,7 +1507,7 @@ Ian's proposal:
 	</div>
 
 <h4 id=attr-substitution>
-''attr()'' Substitution</h4>
+Attribute Value Substitution: the ''attr()'' notation</h4>
 
 	''attr()'' is an [=arbitrary substitution function=],
 	similar to ''var()'',