doc: add default value for the timeout option of http.Agent in http.md

[howiezhao]

Add default value (0) for the timeout option of http.Agent in http.md

[nodejs-github-bot]

Review requested:

• @nodejs/http
• @nodejs/net

[ShogunPanda]

LGTM!

[LiviaMedeiros]

LGTM with shortening the commit message (simply omitting in `http.md` should be sufficient).

[lpinca]

The default value seems to actually be undefined.

// test.js
const http = require('http');

const agent = new http.Agent();

agent.createConnection = function (options) {
  console.log(options);
};

const request = http.request({
  agent,
  host: 'example.com'
});

$ node test.js 
[Object: null prototype] {
  agent: Agent {
    _events: [Object: null prototype] {
      free: [Function (anonymous)],
      newListener: [Function: maybeEnableKeylog]
    },
    _eventsCount: 2,
    _maxListeners: undefined,
    defaultPort: 80,
    protocol: 'http:',
    options: [Object: null prototype] { noDelay: true, path: null },
    requests: [Object: null prototype] {},
    sockets: [Object: null prototype] { 'example.com:80:': [] },
    freeSockets: [Object: null prototype] {},
    keepAliveMsecs: 1000,
    keepAlive: false,
    maxSockets: Infinity,
    maxFreeSockets: 256,
    scheduling: 'lifo',
    maxTotalSockets: Infinity,
    totalSocketCount: 0,
    createConnection: [Function (anonymous)],
    [Symbol(shapeMode)]: false,
    [Symbol(kCapture)]: false
  },
  host: 'example.com',
  port: 80,
  noDelay: true,
  path: null,
  servername: 'example.com',
  _agentKey: 'example.com:80:',
  encoding: null
}