Fix typo in firmware image format documentation (ESPTOOL-1205)

[tyeth]

Unchecked, but seems a sensible change.

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[tyeth]

There's also something weird in the next table, the three values are listed as '0xf', 0, 2 which might be another typo.
Feel free to close this and do a fresh commit with the correct title.

[peterdragun]

I believe that this is correct and explained a couple of lines later, after the table in the note: Flash frequency with value ``0`` can mean either 80MHz or 40MHz based on MSPI clock source mode. Basically, there is a clock divider that will be set in ESP-IDF, which defines the final clock frequency.

Also, please note that these values correspond to ones in the code: https://github.com/espressif/esptool/blob/master/esptool/targets/esp32c6.py#L82-L86

[dobairoland]

I think this could be improved in a way so it won't raise suspicion about being a mistake. For example:

``0`` = 80MHz or 40MHz

[tyeth]

Ah wonderful, thanks folks for the info and suggestion