Changing exception

[adeliofioritto]

In php-graph-sdk/docs/examples/facebook_login.md , looking on group, exception sounds to be like that:

try {
  $accessToken = $helper->getAccessToken();

} catch (\Facebook\Exceptions\FacebookResponseException $e) {
echo "Response Exception: " . $e->getMessage();
exit();
} catch (\Facebook\Exceptions\FacebookSDKException $e) {
echo "SDK Exception: " . $e->getMessage();
exit();
}