doc: fix incorrect vm.createContext usage

In code example `vm.createContext` called with new operator by mistake.
It is not a constructor.

PR-URL: https://github.com/nodejs/node/pull/16059
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>

doc/api/vm.md

@@ -117,7 +117,7 @@ const sandbox = {
 
 const script = new vm.Script('count += 1; name = "kitty";');
 
-const context = new vm.createContext(sandbox);
+const context = vm.createContext(sandbox);
 for (let i = 0; i < 10; ++i) {
   script.runInContext(context);
 }