doc: add require statement in the example

PR-URL: #35554
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

doc/api/v8.md

@@ -102,6 +102,8 @@ 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);
 ```