Add argument with default float value to example
Use io_lib:format/2 instead of float_to_list/1 to display floats

Erlang's list_to_float/1 function will use scientific notation and add
additional less significant digits to the resulting string due to the
internal floating point representation. io_lib:format/2 rounds the
values to better match what the user normally expects. e.g.

> float_to_list(0.66).
"6.60000000000000031086e-01"
> io_lib:format("~w", [0.66]).
["0.66"]