[css-align-3] Fix capitalization

Author: fantasai

css-align-3/Overview.bs

@@ -97,7 +97,7 @@ Introduction</h2>
 	<em>This section (above) is not normative.</em>
 
 <h3 id="placement">
-Module interactions</h3>
+Module Interactions</h3>
 
 	This module adds some new alignment capabilities
 	to the block layout model described in [[!CSS2]] <a href="https://www.w3.org/TR/CSS2/visuren.html">chapters 9</a>