Fix class name used for illegal examples.

Author: dbaron

css3-conditional/Overview.html

@@ -720,7 +720,7 @@ <h2 id=at-supports><span class=secno>6. </span>Feature queries: the
   <div class=example>
    <p>For example, the following rule is not valid:
 
-   <pre class=illegal-example>@supports (transition-property: color) or
+   <pre class=illegal>@supports (transition-property: color) or
           (animation-name: foo) and
           (transform: rotate(10deg)) {
   // ...
@@ -749,7 +749,7 @@ <h2 id=at-supports><span class=secno>6. </span>Feature queries: the
   <div class=example>
    <p>For example, the following rule is not valid:
 
-   <pre class=illegal-example>@supports display: flexbox {
+   <pre class=illegal>@supports display: flexbox {
   // ...
 }</pre>
 

css3-conditional/Overview.src.html

@@ -517,7 +517,7 @@ <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
 
 <div class="example">
 <p>For example, the following rule is not valid:
-<pre class="illegal-example">@supports (transition-property: color) or
+<pre class="illegal">@supports (transition-property: color) or
           (animation-name: foo) and
           (transform: rotate(10deg)) {
   // ...
@@ -540,7 +540,7 @@ <h2 id="at-supports">Feature queries: the '@supports' rule</h2>
 
 <div class="example">
 <p>For example, the following rule is not valid:
-<pre class="illegal-example">@supports display: flexbox {
+<pre class="illegal">@supports display: flexbox {
   // ...
 }</pre>
 <p>Instead, authors must write:</p>