[css-flexbox] Move statement about auto margins to the correct section. No normative changes intended.

Author: tabatkins

css-flexbox-1/Overview.bs

@@ -2612,7 +2612,6 @@ Line Length Determination</h3>
 		<li id='algo-main-container'>
 			<strong>Determine the <a>main size</a> of the flex container</strong>
 			using the rules of the formatting context in which it participates.
-			For this computation, <a value for=margin>auto</a> margins on flex items are treated as ''0''.
 	</ol>
 
 <h3 id='main-sizing'>
@@ -3000,7 +2999,7 @@ Intrinsic Sizes</h3>
 	to produce various types of content-based automatic sizing,
 	such as shrink-to-fit logical widths (which use the ''fit-content'' formula)
 	and content-based logical heights (which use the <a>max-content size</a>).
-
+	For these computations, <a value for=margin>auto</a> margins on flex items are treated as ''0''.
 
 	See [[!CSS-SIZING-3]] for a definition of the terms in this section.