Fixed minor error in example error message in README.
Fixed minor error in example error message in README.

mpflanzer/Oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind