small change to compile on latest Arduino 1.5.7

I was unable to get this library to compile with the latest Arduino (1.5.7) - per this page, it appears as though the compiler changed slightly: http://postwarrior.com/arduino-ethershield-error-prog_char-does-not-name-a-type/

The change here works for me, I'm able to compile and use Arduino to run the example sketch...