doc: point the link aggregation entry at the document that exists

The document was renamed from doc/trunking.md in "Update LAG configuration
registers" and the entry in the README kept the old name, so it leads
nowhere.

README.md

@@ -358,7 +358,7 @@ The following documents give further documentation on specific features of the R
 - [L2 learning](doc/l2.md) 
 - [IGMP (IP-MC streaming)](doc/igmp.md)
 - [SFP+ ports](doc/sfp.md) 
-- [Trunking aka. port aggregation](doc/trunking.md)
+- [Trunking aka. port aggregation](doc/link_aggregation.md)
 - [LACP (802.3ad link aggregation)](doc/lacp.md)
 - [VLAN](doc/vlan.md)
 - [Storm control](doc/storm_control.md)