trivial: fix typo in ieee802154 documentation and add it to index

Signed-off-by: NThadeu Lima de Souza Cascardo <cascardo@holoscopio.com>

Documentation/networking/00-INDEX

@@ -60,6 +60,8 @@ framerelay.txt
 	- info on using Frame Relay/Data Link Connection Identifier (DLCI).
 generic_netlink.txt
 	- info on Generic Netlink
+ieee802154.txt
+	- Linux IEEE 802.15.4 implementation, API and drivers
 ip-sysctl.txt
 	- /proc/sys/net/ipv4/* variables
 ip_dynaddr.txt

Documentation/networking/ieee802154.txt

@@ -69,7 +69,7 @@ We provide an example of simple HardMAC driver at drivers/ieee802154/fakehard.c
 SoftMAC
 =======
 
-We are going to provide intermediate layer impelementing IEEE 802.15.4 MAC
+We are going to provide intermediate layer implementing IEEE 802.15.4 MAC
 in software. This is currently WIP.
 
 See header include/net/ieee802154/mac802154.h and several drivers in