Update Reducers.md to be a bit more succinct (reduxjs#2970)

"describe the fact that *something happened*" can be nothing more than a BEEP that is irregular and inconsistent in it's response to an input, while an action (that is of any use beyond a '*' (all)) is going to describe (as defined by the author) *what happened*, through a series of one or more descriptors/data points.

docs/basics/Reducers.md

@@ -1,6 +1,6 @@
 # Reducers
 
-**Reducers** specify how the application's state changes in response to [actions](./Actions.md) sent to the store. Remember that actions only describe the fact that *something happened*, but don't describe how the application's state changes.
+**Reducers** specify how the application's state changes in response to [actions](./Actions.md) sent to the store. Remember that actions only describe *what happened*, but don't describe how the application's state changes.
 
 ## Designing the State Shape