fix typo in C++11 non-compliance pre-processor error message

// C++11 check

#ifndef LAMMPS_CXX98

#if __cplusplus <= 199711L

  #error LAMMPS is planning to transition to C++11. Do disable this error please use a C++11 compliant compiler, enable C++11 (or later) compliance, or define LAMMPS_CXX98 in your makefile

  #error LAMMPS is planning to transition to C++11. To disable this error please use a C++11 compliant compiler, enable C++11 (or later) compliance, or define LAMMPS_CXX98 in your makefile

#endif

#endif