Update upload-to-s3.php

Erro at line 65!
From:
$dimensions = Mage::getStoreConfig(Ventory_CDN_Model_Config::DIMENSIONS);
To:
$dimensions = Mage::getStoreConfig(MVentory_CDN_Model_Config::DIMENSIONS);
Merge pull request #1 from jmfaraujo/patch-1

Typo fix in upload-to-s3.php