Update hdf5.md

changed line 275 from "d = d_create(fid, "b", Int, ((1000,),(-1,)), "chunk", (100,)) #-1 is equivalent to typemax(Hsize)" to "b = d_create(fid, "b", Int, ((1000,),(-1,)), "chunk", (100,)) #-1 is equivalent to typemax(Hsize)".
The reason is because lines 276 and 277 operates on b instead of d.