Update testenv instructions for c-lightning v0.8.0

This won't actually work without ElementsProject/lightning#3353,
but neither will the instructions currently printed out (LN_ALICE_PATH and
LN_BOB_PATH already include the network subdirectory and `--network` is not
specified), so probably better that way anyway.

bin/testenv.sh

@@ -6,8 +6,8 @@ cat <<EOL
 You can access bitcoind and lightningd via:
 
   $ bitcoin-cli --datadir=$BTC_DIR
-  $ lightning-cli --lightning-dir=$LN_ALICE_PATH
-  $ lightning-cli --lightning-dir=$LN_BOB_PATH
+  $ lightning-cli --rpc-file=$LN_ALICE_PATH/lightning-rpc
+  $ lightning-cli --rpc-file=$LN_BOB_PATH/lightning-rpc
 
 Lightning Charge is available at: