docs(legacy): use the correct binary name when building via docker

docs/legacy/index.md

@@ -11,7 +11,7 @@ cd trezor-firmware
 ./build-docker.sh
 ```
 
-When the build is done, you will find the current firmware in `build/legacy/firmware/trezor.bin`.
+When the build is done, you will find the current firmware in `build/legacy/firmware/firmware.bin`.
 
 ### Running with sudo
 
@@ -103,7 +103,7 @@ Build with `MEMORY_PROTECT=0` or you will get a hard fault on your device.
 
 Switch your device to bootloader mode, then execute:
 ```sh
-trezorctl firmware-update -f build/legacy/firmware/trezor.bin
+trezorctl firmware-update -f build/legacy/firmware/firmware.bin
 ```
 
 ## Combining bootloader and firmware with various `MEMORY_PROTECT` settings, signed/unsigned