Update pygrass_raster.rst

[NikosAlexandris]

In the Introduction to Raster classes, please the remove the incorrect short sentence

Both classes write sequentially.

from

The read access is row wise for RasterRow? and RasterRowIO and additionally cached in the RowIO class. Both classes write sequentially.

Only RasterRow (that is only one class) writes sequentially.

See also: https://trac.osgeo.org/grass/ticket/3766

[neteler]

Fixed in https://trac.osgeo.org/grass/changeset/74313