docs: fix playground link

Author: js2me

docs/introduction/playground.md

@@ -2,7 +2,7 @@
 
 You can try it out in [CodeSandbox](https://codesandbox.io/p/sandbox/nl8kzc).
 
-<iframe src="https://codesandbox.io/embed/nl8kzc?view=editor+%2B+preview&module=%2Fsrc%2Findex.tsx"
+<iframe src="https://codesandbox.io/embed/nl8kzc?view=editor+%2B+preview&module=%2Fsrc%2Fchild.tsx"
   style="width:100%; height: 500px; border:0; border-radius: 4px; overflow:hidden;"
   title="mobx-view-model-example"
   allow="accelerometer; ambient-light-sensor; camera; encrypted-media; geolocation; gyroscope; hid; microphone; midi; payment; usb; vr; xr-spatial-tracking"