[docs] A suggestion to change decimal module docstring

[Alexander Belopolsky]

Not worth a tracker entry, so I am just sending this to docs and Mark.

I find current "(for instance, in binary floating point, 1.00 % 0.1
gives 0.09999999999999995 instead of the expected Decimal("0.00")
returned by decimal floating point)" in the introduction a bit unclear
because neither Decimal class nor it's constructor taking a string has
been introduced by that point.  I think it is better to just use 0.00
for the "expected" value.