Update create-a-subnet.md - fix lock_cost -> burn-cost (#555)

Update create a subnet docs to properly reflect the use of the "burn-cost" command. Current docs incorrectly instruct the user to run "btcli subnet lock_cost" which is not a valid command on the newest version of btcli. rather this command is "btcli subnet burn-cost"

Author: Tom-A-Lynch

docs/subnets/create-a-subnet.md

@@ -95,7 +95,7 @@ If you have not already done so, create Bittensor wallet(s) using the steps desc
 ### Step 2. Get the current price 
 
 ```bash
-btcli subnet lock_cost --subtensor.network test
+btcli subnet burn-cost --subtensor.network test
 ```
 Output:
 ```bash
@@ -146,7 +146,7 @@ If you have not already done so, create Bittensor wallet(s) using the steps desc
 The code below shows how to get the current price of creating a subnet on the mainchain (when the `--subtensor.network` option is not used, then the `btcli` will default to the `mainchain`).
 
 ```bash
-btcli subnet lock_cost
+btcli subnet burn-cost
 ```
 Output:
 ```bash
@@ -171,4 +171,4 @@ Output:
 >> Register subnet? [y/n]: <y/n>        # Select yes (y)
 >> Registering subnet...
 ✅ Registered subnetwork with netuid: 1 # Your subnet netuid will show here, save this for later.
-```
+```