docs: networking: packet_mmap: fix old config reference

Before commit 889b8f96

 ("packet: Kill CONFIG_PACKET_MMAP.") there
used to be a CONFIG_PACKET_MMAP config symbol that depended on
CONFIG_PACKET. The text still implies that PACKET_MMAP can be disabled.
Remove that from the text, as well as reference to old kernel versions.

Also, drop reference to broken link to information for pre 2.6.5
kernels.

Make a slight working improvement (s/In/On/) while at it.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Link: https://lore.kernel.org/r/80089f3783372c8fd7833f28ce774a171b2ef252.1609232919.git.baruch@tkos.co.il
Signed-off-by: Jakub Kicinski <kuba@kernel.org>