message: minor fix to field-diff
diff: // Compile with: make field-diff
  //
  // Sample runs:
  -// compare
  -// compare -m1 triple-pt-1.mesh -s1 triple-pt-1.gf -m2 triple-pt-2.mesh -s2 triple-pt-1.gf -p 200
  +// field-diff
  +// field-diff -m1 triple-pt-1.mesh -s1 triple-pt-1.gf -m2 triple-pt-2.mesh -s2 triple-pt-1.gf -p 200
  #include "mfem.hpp"
  #include <fstream>
  