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>

MAINTAINERS

@@ -176,7 +176,7 @@ L:	linux-wpan@vger.kernel.org
 S:	Maintained
 F:	net/6lowpan/
 F:	include/net/6lowpan.h
-F:	Documentation/networking/6lowpan.txt
+F:	Documentation/networking/6lowpan.rst
 
 6PACK NETWORK DRIVER FOR AX.25
 M:	Andreas Koensgen <ajk@comnets.uni-bremen.de>