Update README.md

README.md

-GTXE2_CHANNEL_GPL.v is a non-synthesizable open-source replacement for a propriate xilinx's primitive GTXE2_CHANNEL.v
+GTXE2_CHANNEL_GPL.v is a non-synthesizable free software replacement for a propriate xilinx's primitive GTXE2_CHANNEL.v
 For now only basic features are implemented : OOB, comma aligment, 10-8 encoding/decoding, clocking scheme. 
 Still, that is enough to run SATA protocol implementation simulation.