Clarify that the negative sign must remain when shortening -1n

Author: fantasai

selectors3/Overview.html

@@ -1949,8 +1949,8 @@ <h5 id=nth-child-pseudo><span class=secno>6.6.5.2. </span>:nth-child()
 foo:nth-child(5)      /* same */</pre>
   </div>
 
-  <p>When <var>a</var>=1, or <var>a</var>=-1, the number may be omitted from
-   the rule.
+  <p>When <var>a</var>=1, or <var>a</var>=-1, the <code>1</code> may be omitted
+   from the rule.
 
   <div class=example>
    <p>Examples:</p>

selectors3/Overview.src.html

@@ -1488,7 +1488,7 @@ <h5 id=nth-child-pseudo>:nth-child() pseudo-class</h5>
 foo:nth-child(5)      /* same */</pre>
 </div>
 
-<p>When <var>a</var>=1, or <var>a</var>=-1, the number may be
+<p>When <var>a</var>=1, or <var>a</var>=-1, the <code>1</code> may be
 omitted from the rule.
 
 <div class="example">