fix: fixed vuejs#220 on injection reactivity section

Author: NataliaTepluhina

src/guide/composition-api-provide-inject.md

@@ -74,7 +74,7 @@ setup() {
   const year = ref('2020')
 
   provide('book', book)
-  provide('year', book)
+  provide('year', year)
 }
 
 // in consumer