title: convert NSArray into NSSortDescriptor
body: <p>I am new to iOS. I have a warning message and I can t solve it:)
  I have the following code:</p>
  
  <pre><code>NSSortDescriptor *sortDescriptorFirst = [NSSortDescriptor sortDescriptorWithKey:@"test" ascending:YES];
  NSSortDescriptor *descriptor=[NSArray arrayWithObjects:sortDescriptorFirst,nil];
  NSArray *sortedArray =[sections sortedArrayUsingDescriptors:@[descriptor]];
  </code></pre>
  
  <p>At line 2 I have a warning:</p>
  
  <pre><code>Incompatible pointer types initializing NSSortDescriptor with an expression of NSArray
  </code></pre>
  
  <p>How to solve this?</p>