Update read_hdf default mode to "r" (#7039)
Update read_hdf default mode to "r" (#7039)

* Update hdf.py

Changed read_hdf function (line 422) to have default mode as 'r' instead of 'a', similar to pandas. See https://github.com/dask/dask/issues/7022#issuecomment-756178918 for elaboration

* Update docstring

Co-authored-by: James Bourbeau <d47a96e8b557881d8d6a5263afaffb9cba464dc8@gmail.com>
blaze/dask,jakirkham/dask,blaze/dask,dask/dask,jakirkham/dask,dask/dask