[css-align] Rename terminology section because other sections also define terms, p=dbaron, fixes w3c#1451.

Author: fantasai

css-align/Overview.bs

@@ -1598,7 +1598,7 @@ Determining the Baselines of a Box</h2>
 	ISSUE: This reflects the latest CSS2.1 errata, however see also discussion of an <a href="https://github.com/w3c/csswg-drafts/issues/766">alternate solution</a> that was previously drafted here.
 
 <h3 id='baseline-terms'>
-Baseline Alignment Terminology</h3>
+Baseline Alignment Grouping</h3>
 
 	A <dfn export>baseline-sharing group</dfn>
 	is composed of boxes that participate in baseline alignment together.