Mark some errata with reprint numbers

Author: marijnh

html/errata.html

@@ -78,7 +78,7 @@ <h2>Chapter 14</h2>
 
 <h2>Chapter 15</h2>
 
-<p><strong>Page 255</strong> <em>Mouse Motion</em>: The second
+<p><strong>Page 255</strong> (6th) <em>Mouse Motion</em>: The second
 paragraph on the page claims that you can isolate a button from
 the <code>buttons</code> bitmask with the remainder operator, but that
 doesn't really work (you'd also need a division and a floor, which
@@ -93,11 +93,11 @@ <h2>Chapter 15</h2>
 
 <h2>Chapter 16</h2>
 
-<p><strong>Page 271</strong> <em>Actors</em>: Near the bottom of the
+<p><strong>Page 271</strong> (7th) <em>Actors</em>: Near the bottom of the
 page, the book incorrectly uses the term <em>phase</em> (of a sinus
 wave) where it should say <em>period</em>.</p>
 
-<p><strong>Page 278</strong> <em>Motion and Collision</em>: The code
+<p><strong>Page 278</strong> (7th) <em>Motion and Collision</em>: The code
 that defines the <code>touches</code> method unintentionally (though
 harmlessly) used <code>var</code> instead of <code>let</code> to
 define variables.</p>