Fix naming of tracking function argument in reaction

[MaikuB]

Noticed that the argument in reaction was named as predicate when it doesn't return a boolean value. Changed this to fn though we may want to change this something else. Alternatives I can think of are trackingFunction/trackingFn or dataFunction/dataFn

Sidenote: suggest agreeing on a standard in the API docs. I noticed there are cases where functions are referred to as <function> and other places use <function>(). I would suggest the former and it's also the approach used in the Flutter docs

[codecov]

Codecov Report

Merging #398 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #398   +/-   ##
=======================================
  Coverage   97.87%   97.87%           
=======================================
  Files          46       46           
  Lines        1644     1644           
=======================================
  Hits         1609     1609           
  Misses         35       35

Flag	Coverage Δ
#flutter_mobx	100% <ø> (ø)	⬆️
#mobx	97.77% <100%> (ø)	⬆️
#mobx_codegen	100% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
mobx/lib/src/api/reaction.dart	100% <100%> (ø)	⬆️
mobx/lib/src/core/reaction_helper.dart	96.36% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a08a9a5...c8a68ee. Read the comment docs.

[pavanpodila]

Sidenote: suggest agreeing on a standard in the API docs. I noticed there are cases where functions are referred to as <function> and other places use <function>(). I would suggest the former and it's also the approach used in the Flutter docs

Agree with the consistency of style. We can do it the way Flutter docs do it. Sounds like a new PR :-)