Update name of property initializer proposal (#10812)

The proposal for property initializers is called [Public Class Fields](https://tc39.github.io/proposal-class-public-fields/) now (part of the combined [Class Fields](https://github.com/tc39/proposal-class-fields) proposal).

docs/handling-events.md

@@ -97,7 +97,7 @@ You have to be careful about the meaning of `this` in JSX callbacks. In JavaScri
 
 This is not React-specific behavior; it is a part of [how functions work in JavaScript](https://www.smashingmagazine.com/2014/01/understanding-javascript-function-prototype-bind/). Generally, if you refer to a method without `()` after it, such as `onClick={this.handleClick}`, you should bind that method.
 
-If calling `bind` annoys you, there are two ways you can get around this. If you are using the experimental [property initializer syntax](https://babeljs.io/docs/plugins/transform-class-properties/), you can use property initializers to correctly bind callbacks:
+If calling `bind` annoys you, there are two ways you can get around this. If you are using the experimental [public class fields syntax](https://babeljs.io/docs/plugins/transform-class-properties/), you can use class fields to correctly bind callbacks:
 
 ```js{2-6}
 class LoggingButton extends React.Component {
@@ -119,7 +119,7 @@ class LoggingButton extends React.Component {
 
 This syntax is enabled by default in [Create React App](https://github.com/facebookincubator/create-react-app).
 
-If you aren't using property initializer syntax, you can use an [arrow function](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions) in the callback:
+If you aren't using class fields syntax, you can use an [arrow function](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions) in the callback:
 
 ```js{7-9}
 class LoggingButton extends React.Component {
@@ -138,4 +138,4 @@ class LoggingButton extends React.Component {
 }
 ```
 
-The problem with this syntax is that a different callback is created each time the `LoggingButton` renders. In most cases, this is fine. However, if this callback is passed as a prop to lower components, those components might do an extra re-rendering. We generally recommend binding in the constructor or using the property initializer syntax, to avoid this sort of performance problem.
+The problem with this syntax is that a different callback is created each time the `LoggingButton` renders. In most cases, this is fine. However, if this callback is passed as a prop to lower components, those components might do an extra re-rendering. We generally recommend binding in the constructor or using the class fields syntax, to avoid this sort of performance problem.