Fix README mixin usage (#292)

Nargonath · ktsn · commit 7ecde38fd4f8 · 2018-12-04T23:41:43.000+08:00
The mixin is exported as a named export but used as a default export.
diff --git a/README.md b/README.md
@@ -89,7 +89,7 @@ import Component from 'vue-class-component'
 
 // You can declare a mixin as the same style as components.
 @Component
-export class MyMixin extends Vue {
+export default class MyMixin extends Vue {
   mixinValue = 'Hello'
 }
 ```

Original file line number	Diff line number	Diff line change
`@@ -89,7 +89,7 @@ import Component from 'vue-class-component'`
`89`	`89`
`90`	`90`	`// You can declare a mixin as the same style as components.`
`91`	`91`	`@Component`
`92`		`-export class MyMixin extends Vue {`
	`92`	`+export default class MyMixin extends Vue {`
`93`	`93`	`mixinValue = 'Hello'`
`94`	`94`	`}`
`95`	`95`	```