Fix logic in "set the dictionary members"

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=29719

Author: zcorpan

geometry/Overview.bs

@@ -662,7 +662,7 @@ To <dfn>set the dictionary members</dfn> <var>a</var> and <var>b</var> of a {{DO
 <ol>
   <li>If <var>a</var> is present and <var>b</var> is not present, set the <var>b</var> dictionary member to the value of <var>a</var>.
   <li>Otherwise, if <var>b</var> is present and <var>a</var> is not present, set the <var>a</var> dictionary member to the value of <var>b</var>.
-  <li>Otherwise, set <var>a</var> and <var>b</var> dictionary members to <var>fallback</var>.
+  <li>Otherwise, if neither <var>a</var> or <var>b</var> are present, set <var>a</var> and <var>b</var> dictionary members to <var>fallback</var>.
 </ol>
 
 <h3 id='dommatrix-create'>Creating DOMMatrixReadOnly and DOMMatrix objects</h3>