tools: update package.json `engine` field

PR-URL: https://github.com/nodejs/node/pull/14165
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/doc/package.json

@@ -4,7 +4,7 @@
   "description": "Internal tool for generating Node.js API docs",
   "version": "0.0.0",
   "engines": {
-    "node": ">=0.6.10"
+    "node": ">=6"
   },
   "dependencies": {
     "marked": "^0.3.5",