Correct typo in getting Carrier name example

The method for retrieving the name of the carrier
for the given phone number is named `getNameForNumber`
  and not `getDescriptionForNumber`

See
[PhoneNumberToCarrierMapper.php](https://github.com/sergeylukin/libphonenumber-for-php/blob/master/src/libphonenumber/PhoneNumberToCarrierMapper.php)
for reference
Merge pull request #29 from sergeylukin/readme-carrier-method

Correct typo in getting Carrier name example in README