doc: fix tmp36 conversion algorithm

[fivdi]

Currently the TMP36 conversion formula is documented in pins.md as:

let temperature = (Analog.read(0) / 1024) * 330;

The maximum value returned by Analog.read(0) is 1023 so it should be divide by 1023 rather than by 1024. The Adafruit tutorial linked to is also incorrect here. The TPM36 also has a 500mV or 50°C offset that needs to be taken into account. The correct conversion formula is therefore:

let temperature = (Analog.read(0) / 1023) * 330 - 50;