fix one error in compute_forces_acoustic.f90

--error with using LDDRK in acoustic simulation with PML
--modify one variable name in specfem2D.F90 with changing rmemory_potential_acoust_LDDRK to rmemory_potential_acoustic_LDDRK
Merge pull request #3 from xiezhinan/devel

fix one error in compute_forces_acoustic.f90