[css-table] Made a small clarifying edit to determining CAPMIN

Author: gregwhitworth

css-tables-3/Overview.bs

@@ -647,7 +647,7 @@ spec:css21; type:property; text:max-width
 		<p>To layout a table, user agents must apply the following actions:
 			<ol>
 				<li><b>If the table has at least one caption, compute their minimum width.</b><br/>
-					This is done by computing their min-content width (including margins, borders and paddings) 
+					This is done by computing the caption's min-content width (including margins, borders and paddings) 
 					under the assumption their parent has a zero width and overflow:visible.
 					The largest such width will be known as <a href="#capmin">CAPMIN</a>.