net: ethernet: Fix double word "the the" in eth.c

This patch fix double word "the the" in
Documentation/DocBook/networking/API-eth-get-headlen.html
Documentation/DocBook/networking/netdev.html
Documentation/DocBook/networking.xml

These files are generated from comment in source,
so I have to fix comment in net/ethernet/eth.c.
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ethernet/eth.c

@@ -114,7 +114,7 @@ int eth_header(struct sk_buff *skb, struct net_device *dev,
 EXPORT_SYMBOL(eth_header);
 
 /**
- * eth_get_headlen - determine the the length of header for an ethernet frame
+ * eth_get_headlen - determine the length of header for an ethernet frame
  * @data: pointer to start of frame
  * @len: total length of frame
  *