doc: add require statement in the example

Author: PoojaDurgad

doc/api/v8.md

@@ -104,6 +104,8 @@ Chrome DevTools. The JSON schema is undocumented and specific to the
 V8 engine, and may change from one version of V8 to the next.
 
 ```js
+// Print heap snapshot to the console
+const v8 = require('v8');
 const stream = v8.getHeapSnapshot();
 stream.pipe(process.stdout);
 ```