Change name of setState updater first arg to 'state'

[mmcgahan]

Followup from tweet about setState updater docs, with support from @gaearon:

Possible confusion in React docs for setState updater pattern - calling first arg prevState seems odd because it could actually be a future value of state relative to this.state. Should it just be called state and align with getDerivedStateFromProps args?

https://twitter.com/mikemcgahan/status/1037074327269568512

The reference to 'previous state' is ambiguous in the 'updater' context, so this PR changes the suggested prevState arg name to state. This change helps conceptually align the 'updater' function with the behavior of getDerivedStateFromProps - both are pure functions that take a 'current' state arg and return partialState updates.

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[karlli1976]

It's glad to know I'm not alone :)

Actually I found the code on Try It On CodePen still uses prevState, which causes me a bit confused: according to the context, shouldn't it be state?

And I found if I change prevState to prevMyState, the code still works.

this.setState(prevMyState => ({
      showWarning: !prevMyState.showWarning
    }));

[mmcgahan]

@karlli1976 yeah, you can technically name it whatever you want - it won't break the code, but it may mislead people who read prevState as referring to some sort of cached or 'past' state. I can't update the CodePen code, but it's probably not critical