[css-conditional-3] Fix comment syntax in example.

Fixes #3994.

css-conditional-3/Overview.bs

@@ -365,22 +365,22 @@ For example, the following rule is not valid:
 @supports (transition-property: color) or
           (animation-name: foo) and
           (transform: rotate(10deg)) {
-  // ...
+  /* ... */
 }
 </pre>
 Instead, authors must write one of the following:
 <pre>
 @supports ((transition-property: color) or
            (animation-name: foo)) and
           (transform: rotate(10deg)) {
-  // ...
+  /* ... */
 }
 </pre>
 <pre>
 @supports (transition-property: color) or
           ((animation-name: foo) and
            (transform: rotate(10deg))) {
-  // ...
+  /* ... */
 }
 </pre>
 </div>
@@ -392,13 +392,13 @@ when it is the only thing in the expression.
 For example, the following rule is not valid:
 <pre class="illegal">
 @supports display: flex {
-  // ...
+  /* ... */
 }
 </pre>
 Instead, authors must write:
 <pre>
 @supports (display: flex) {
-  // ...
+  /* ... */
 }
 </pre>
 </div>
@@ -412,7 +412,7 @@ authoring tools.
 For example, authors may write:
 <pre>
 @supports ((display: flex)) {
-  // ...
+  /* ... */
 }
 </pre>
 </div>
@@ -424,7 +424,7 @@ though it won't change the validity of the declaration.
 For example, the following rule is valid:
 <pre>
 @supports (display: flex !important) {
-  // ...
+  /* ... */
 }
 </pre>
 </div>