title: Update document in array in the collection using $set and positional $ operator
body: <p><strong>Collection :</strong> </p>
  
  <pre><code>{
      "shopping_list": [
          {
              "date": 22,
              "drinks": [1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000],
              "year": 2016,
              "month": 11
          },
          {
              "date": 23,
              "drinks": [1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000],
              "year": 2016,
              "month": 11
          }
      ],
      "password": "user",
      "date_signup": "10-11-2016",
      "name": "User",
      "email": "user@user.com"
  }
  </code></pre>
  
  <p><strong>The code I made :</strong> </p>
  
  <pre><code>data_1 = [1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000, 1000]
  data_2 = [2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000, 2000]
  con.update({"email": "user@user.com",
              "shopping_list.date": 23,
              "shopping_list.month": 11,
              "shopping_list.year": 2016},
             {"$set": {"shopping_list.$.drinks": data_2}})
  </code></pre>
  
  <p><strong>Tried this too (didn't work) :</strong></p>
  
  <pre><code>con.update({"email": "user@user.com",
              "shopping_list.date": {"$eq": 23},
              "shopping_list.month": {"$eq": 11},
              "shopping_list.year": {"$eq": 2016}},
             {"$set": {"shopping_list.$.drinks": data_2}})
  </code></pre>
  
  <p>With my code I'm targeting the <code>shopping_list</code>in the second index of the array using <code>$set</code>. But nothing change after I run it. What is wrong with my code?</p>