corrected documentation for php send inline images, was missing an array...

corrected documentation for php send inline images, was missing an array in the example.

incorrect example 
```php
 $result = $mgClient->sendMessage($domain, array(
    'from'    => 'Excited User <me@samples.mailgun.org>',
   // ...
), array(
    'inline' => '@/path/to/test.jpg'
));
```
The documentation updated to reflect this change 

```php
$result = $mgClient->sendMessage($domain, array(
    'from'    => 'Excited User <me@samples.mailgun.org>',
    // ...
), array(
    'inline' => array('@/path/to/test.jpg')
));
```