docs(esp8266): change boot select pin to IO0 (ESPTOOL-968)

[chromalock]

Quick background: I am building a simple breakout board for an esp-wroom-02 module, and noticed what seems to be an error in the current esptool.py ESP8266 "Boot Mode Selection" documentation.

Notably, the boot mode documentation lists GPIO0 in some sections correctly, but GPIO9 later in reference to the same thing.

Specifically, it lists GPIO9 as the boot mode selection pin that must be connected to DTR, when both documentation for the esp-wroom-02 module and esp8266EX microcontroller list GPIO0 as the boot mode selection pin.

Hopefully this is not just a misunderstanding with the documentation; I've been searching for that mythical GPIO9 on the esp8266 for a while! 🤣

This change fixes the following bug(s):

Error in the "Boot Mode Selection" documentation for the ESP8266, where it claims GPIO9 is the selection pin instead of GPIO0

I have tested this change with the following hardware & software combinations:

NO TESTING

I have run the esptool.py automated integration tests with this change and the above hardware:

NO TESTING

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[peterdragun]

Hi @chromalock
I am sorry for any confusion that this issue caused and thank you for the contribution!
LGTM