Fix slow demo preview height (#8517)

Co-authored-by: Tuna <tuna@MacBookPro.station>

Author: TunaDev0

src/content/reference/react/useDeferredValue.md

@@ -706,6 +706,8 @@ export default SlowList;
 ```css
 .items {
   padding: 0;
+  max-height: 300px;
+  overflow: auto;
 }
 
 .item {
@@ -783,6 +785,8 @@ export default SlowList;
 ```css
 .items {
   padding: 0;
+  max-height: 300px;
+  overflow: auto;
 }
 
 .item {

src/content/reference/react/useTransition.md

@@ -736,6 +736,10 @@ export default function ContactTab() {
 button { margin-right: 10px }
 b { display: inline-block; margin-right: 10px; }
 .pending { color: #777; }
+.items {
+  max-height: 300px;
+  overflow: auto;
+}
 ```
 
 </Sandpack>
@@ -891,6 +895,10 @@ export default function ContactTab() {
 button { margin-right: 10px }
 b { display: inline-block; margin-right: 10px; }
 .pending { color: #777; }
+.items {
+  max-height: 300px;
+  overflow: auto;
+}
 ```
 
 </Sandpack>