Use const instead of var (ES6 best practices)

docs/05-reusable-components.it-IT.md

@@ -249,7 +249,7 @@ ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 Oppure usando la nuova sintassi freccia di ES6:
 
 ```javascript
-var HelloMessage = (props) => <div>Ciao {props.name}</div>;
+const HelloMessage = (props) => <div>Ciao {props.name}</div>;
 ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 ```
 

docs/05-reusable-components.ko-KR.md

@@ -244,7 +244,7 @@ ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 아니면 ES6의 화살표 문법을 사용할 수 있습니다.
 
 ```javascript
-var HelloMessage = (props) => <div>Hello {props.name}</div>;
+const HelloMessage = (props) => <div>Hello {props.name}</div>;
 ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 ```
 

docs/05-reusable-components.md

@@ -243,7 +243,7 @@ ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 Or using the new ES6 arrow syntax:
 
 ```javascript
-var HelloMessage = (props) => <div>Hello {props.name}</div>;
+const HelloMessage = (props) => <div>Hello {props.name}</div>;
 ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 ```
 

docs/05-reusable-components.zh-CN.md

@@ -243,7 +243,7 @@ ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 或者使用新的ES6箭头函数:
 
 ```javascript
-var HelloMessage = (props) => <div>Hello {props.name}</div>;
+const HelloMessage = (props) => <div>Hello {props.name}</div>;
 ReactDOM.render(<HelloMessage name="Sebastian" />, mountNode);
 ```