doc - fix formula for calculating the corrected DAC value
In section 5.2.2, the documentation says that the formula for calculating the corrected DAC value is:
c_val = ((val + offset) * gain/0x8000) - 0x8000
However, in bipolar mode, according the manual, the DAC encodes zero as 0x8000, vref as 0xffff and -vref as 0x0000.
Therefore, the formula should be:
c_val = ((val + offset) * gain/0x8000) + 0x8000
Note that both the PTS and the driver use the correct formula, it's just the documentation that has it wrong.