message: Fix duplicate in example 15 docs.
diff: // mpirun -np 4 ex15p -est 0 -e 1e-4
  // mpirun -np 4 ex15p -est 1 -e 1e-6
  // mpirun -np 4 ex15p -est 1 -o 3 -tf 0.3
  -// mpirun -np 4 ex15p -est 1 -o 3 -tf 0.3
  // mpirun -np 4 ex15p -est 2 -o 2
  //
  // Description: Building on Example 6, this example demonstrates dynamic AMR.
  