Small documentation error for function request_uri

Changed the "request_uri()" in the comment for function request_uri to "$_SERVER['REQUEST_URI']" in includes/bootstrap.inc

Original was :
/**
* Since request_uri() is only available on Apache, we generate an
* equivalent using other environment variables.
*/

Modified :
/**
* Since $_SERVER['REQUEST_URI'] is only available on Apache, we generate an
* equivalent using other environment variables.
*/