fix typo

Author: maclobdell

docs/porting/custom-target-porting.md

@@ -242,7 +242,7 @@ Assuming you have Mbed Command Line Interface (CLI) installed, follow these step
   For the `ImaginaryBoard` based on `DISCO-L475VG-IOT01A`, run this command.
 
   ```
-  mbedls --mock 0764:AMAZINGBOARD
+  mbedls --mock 0764:IMAGINARYBOARD
   ```
    
    <span class="notes">**Note:** Contact the Arm Mbed team to get a unique ID if you plan to have your board coexist with other Mbed Enabled boards while running automated tests.</span>