title: Laravel DB::table update record
body: <p>I have this query</p>
  
  <pre><code> DB::table('product_options')
          -&gt;where('product_id', $id)
          -&gt;where('size', $selected_size)
          -&gt;update(array('stock' =&gt; WHAT TO PUT HERE));
  </code></pre>
  
  <p>In the update part where I've put WHAT TO UPDATE, what should I put here to decrease the number by 1?</p>