Move legacy summary section up a level

compiler/snowman.tt

@@ -1337,8 +1337,6 @@ script can be compiled to work with any of the three encoding schemes.
 
 <h2>13 Legacy Features</h2>
 
-<h3>13.1 hex and decimal</h3>
-
 <p>
 This section documents features of Snowball for which there's a strongly
 preferred alternative.  They're still supported for compatibility with
@@ -1347,6 +1345,8 @@ We document them here so that their meaning in existing code can be
 understood, and especially to aid updating to the preferred alternatives.
 </p>
 
+<h3>13.1 hex and decimal</h3>
+
 <p>
 In a &nbsp;<code>stringdef</code>&nbsp;, string may be preceded by the word &nbsp;<code>hex</code>,
 or the word &nbsp;<code>decimal</code>.  This was how non-ASCII characters