doc/source/firmware: fix description of WRITTEN_FW_SIZE

Signed-off-by: Adam Wujek <dev_public@wujek.eu>

doc/source/firmware.rst

@@ -443,8 +443,8 @@ like a regular command.
 | Transaction type: **Word write**
 | Data length: **2**
 
-Before writing a new FW binary through the bootloader, its size in
-bytes has to be given using this command.
+Before writing a new FW binary through the bootloader, its size in bytes
+divided by 8 has to be given using this command.
 
 .. _WRITTEN_FW_BLOCK: