docs: fix typo in component-provide-inject.md

vuejs/docs@3804d0e#diff-e2d6d7ad2f53f101c031a6a845ead5917445c9a7745f832ecb079d8d319c9258

Author: naokie

src/guide/component-provide-inject.md

@@ -59,7 +59,7 @@ app.component('todo-list', {
     }
   },
   provide: {
-    todoLength: this.todos.length // this will result in error 'Cannot read property 'length' of undefined`
+    todoLength: this.todos.length // this will result in error `Cannot read property 'length' of undefined`
   },
   template: `
     ...