Minor editorial tweak to the flex() grammar to avoid implying omittability twice.

Author: tabatkins

css3-flexbox/Overview.html

@@ -1084,7 +1084,7 @@ <h2 id=flexibility><span class=secno>4. </span> Flexibility: the
    class=css>flex()</code>&rsquo; notation is:
 
   <pre class=prod><dfn
-   id=ltflex>&lt;flex></dfn> = flex( [ &lt;pos-flex> &lt;neg-flex>? ]? || &lt;preferred-size>? )</pre>
+   id=ltflex>&lt;flex></dfn> = flex( [ &lt;pos-flex> &lt;neg-flex>? ]? &amp;&amp; &lt;preferred-size>? )</pre>
 
   <p><code>&lt;pos-flex></code> and <code>&lt;neg-flex></code> are
    non-negative <code>&lt;numbers>s</code>, while

css3-flexbox/Overview.src.html

@@ -509,7 +509,7 @@ <h2 id='flexibility'>
 
 	<p>The ''flex()'' notation is used to specify the parameters of a <dfn title="flexible length|flexible lengths|flexible length's">flexible length</dfn>: the <dfn id="positive-flexibility" title="positive flexibility">positive</dfn> and <dfn>negative flexibility</dfn>, and the <dfn>preferred size</dfn>.  The syntax of the ''flex()'' notation is:</p>
 
-	<pre class=prod><dfn>&lt;flex></dfn> = flex( [ &lt;pos-flex> &lt;neg-flex>? ]? || &lt;preferred-size>? )</pre>
+	<pre class=prod><dfn>&lt;flex></dfn> = flex( [ &lt;pos-flex> &lt;neg-flex>? ]? &amp;&amp; &lt;preferred-size>? )</pre>
 
 	<p><code>&lt;pos-flex></code> and <code>&lt;neg-flex></code> are non-negative <code>&lt;numbers>s</code>, while <code>&lt;preferred-size></code> is any value (other than another ''&lt;flex>'') that would be valid in the 'width' or 'height' property in which the notation is used.  If the <code>&lt;preferred-size></code> is ''0'', it <strong>must</strong> be specified with a unit (like ''0px'') to avoid ambiguity; unitless zero will either be interpreted as as one of the flexibilities, or is a syntax error.</p>