Update the playpen link for code in the rust book to agree with rustdoc's new style

Author: mdinger

src/doc/rust.css

@@ -338,14 +338,17 @@ table th {
 
 .rusttest { display: none; }
 pre.rust { position: relative; }
-.test-arrow {
+a.test-arrow {
     display: inline-block;
     position: absolute;
-    top: 0;
-    right: 10px;
-    font-size: 150%;
-    -webkit-transform: scaleX(-1);
-    transform: scaleX(-1);
+
+    background-color: #4e8bca;
+    color: #f5f5f5;
+    padding: 5px 10px 5px 10px;
+    border-radius: 5px;
+    font-size: 130%;
+    top: 5px;
+    right: 5px;
 }
 
 .unstable-feature {