[css-ruby] tweak phrasing

Author: frivoal

css-ruby-1/Overview.bs

@@ -38,7 +38,7 @@ Introduction</h2>
 <h3 id="ruby-def">
 What is ruby?</h3>
 
-	<em>This section is not normative.</em>
+	<em>This subsection is not normative.</em>
 
 	<dfn export>Ruby</dfn> is the commonly-used name for a run of text
 	that appears alongside another run of text (referred to as the “base”)
@@ -124,7 +124,7 @@ Value Definitions</h3>
 <h3 id="diagram-conventions">
 Diagram conventions</h3>
 
-	<em>This section is not normative.</em>
+	<em>This subsection is not normative.</em>
 
 	Many typographical conventions in East Asian typography depend
 	on whether the character rendered is wide (CJK) or narrow (non-CJK).