Fixed wrong statement in Sqlserver class header

http://www.php.net/manual/en/ref.pdo-sqlsrv.php

--> PDO_SQLSRV is a driver that implements the PHP Data Objects (PDO) interface to enable access from PHP to MS SQL Server (starting with SQL Server 2005) and SQL Azure databases.
Merge pull request #1118 from ravage84/patch-1

Fixed wrong statement in Sqlserver class header