BOLFI docs: Change acq_noise_var from list to dict

[bendudson]

Summary:

Fixes the RTD BOLFI example, by changing the value of acq_noise_var passed to elfi.BOLFI from a list to a dict.

Passing a list as the acq_noise_var argument of elfi.BOLFI in the RTD example results in an error:

ValueError: Either acquisition noise is a float or it is a dictionary of floats defining variance for each parameter dimension.