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

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
diff --git a/Documentation/networking/ieee802154.txt b/Documentation/networking/ieee802154.txt
index a0280ad..1d4ed66 100644
--- a/Documentation/networking/ieee802154.txt
+++ b/Documentation/networking/ieee802154.txt
@@ -69,7 +69,7 @@
 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