Update TESTING_GNB_W_COTS_UE.md

doc/TESTING_GNB_W_COTS_UE.md

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 ## Configuration Overview:
 
 * Non Standalone (NSA) configuration  : initial Control Plane established between UE and RAN eNB, then User Plane established between UE and gNB, Core network is 4G based supporting rel 15