docs(image-format): Improve clarity about flash frequency on esp32-c6

Closes #1135

Author: peterdragun

docs/en/advanced-topics/firmware-image-format.rst

@@ -94,7 +94,7 @@ The image header is 8 bytes long:
     +--------+------------------------------------------------------------------------------------------------+
     | 3      | High four bits - Flash size (``0`` = 1MB, ``1`` = 2MB, ``2`` = 4MB, ``3`` = 8MB, ``4`` = 16MB) |
     |        |                                                                                                |
-    |        | Low four bits - Flash frequency (``0`` = 80MHz, ``0`` = 40MHz, ``2`` = 20MHz)                  |
+    |        | Low four bits - Flash frequency (``0`` = 80MHz or 40MHz, ``2`` = 20MHz)                        |
     +--------+------------------------------------------------------------------------------------------------+
     | 4-7    | Entry point address                                                                            |
     +--------+------------------------------------------------------------------------------------------------+