Add padding around flutter api docs search input (#44980)

dev/docs/assets/overrides.css

@@ -107,7 +107,7 @@ code {
   background-image: none;
   border: 1px solid #ccc;
   border-radius: 2px;
-  padding: 4px 6px;
+  padding: 14px 6px;
   font-size: 15px;
 }