MAINTAINERS: adjust to 6lowpan doc ReST conversion

Commit 107db7ec

 ("docs: networking: convert 6lowpan.txt to ReST")
renamed 6lowpan.txt to 6lowpan.rst for the ReST conversion.

Since then, ./scripts/get_maintainer.pl --self-test complains:

  warning: no file matches F: Documentation/networking/6lowpan.txt

Adjust 6LOWPAN GENERIC (BTLE/IEEE 802.15.4) entry in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>