doc: use arrow function

PR-URL: #24590
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/v8.md

@@ -156,7 +156,7 @@ Usage:
 // Print GC events to stdout for one minute.
 const v8 = require('v8');
 v8.setFlagsFromString('--trace_gc');
-setTimeout(function() { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
+setTimeout(() => { v8.setFlagsFromString('--notrace_gc'); }, 60e3);
 ```
 
 ## Serialization API