net: update net_dim documentation after rename

Commit 8960b389 ("linux/dim: Rename externally used net_dim
members") renamed the net_dim API, removing the "net_" prefix from the
structures and functions. The patch didn't update the net_dim.txt
documentation file.

Fix the documentation so that its examples match the current code.

Fixes: 8960b389 ("linux/dim: Rename externally used net_dim members", 2019-06-25)
Fixes: c002bd52 ("linux/dim: Rename externally exposed macros", 2019-06-25)
Fixes: 4f75da36 ("linux/dim: Move implementation to .c files")
Cc: Tal Gilboa <talgi@mellanox.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>