Fix another deprecation warning on missing parenthesis

[sabiwara]

Hi!

This the same fix as https://github.com/bencheeorg/benchee/commit/2d56fba7e9df361943e1342889c816cc2c6056f4.

It fixes the following warning:

warning: using map.field notation (without parentheses) to invoke function Benchee.Conversion.Count.base_unit() is deprecated, you must add parentheses instead: remote.function()
  (benchee 1.2.0) lib/benchee/conversion/scale.ex:165: Benchee.Conversion.Scale.do_best_unit/3
  (benchee 1.2.0) lib/benchee/conversion.ex:63: Benchee.Conversion.units/2
...

I think there is no occurrence left.

Cheers!

[PragTob]

"If I had this error once why do we have it again" "oh, I didn't add a test against elixir main"

._.

Thanks a lot @sabiwara !