MAINTAINERS: adjust to 6lowpan doc ReST conversion

Commit 107db7e ("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 <[email protected]>
Signed-off-by: Marcel Holtmann <[email protected]>

Author: bulwahn

MAINTAINERS

@@ -176,7 +176,7 @@ L:	[email protected]
 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 <[email protected]>