chore: fix comment grammar mistake (vuejs#7739)

Author: LarsStegman

src/core/observer/index.js

@@ -27,10 +27,10 @@ export const observerState = {
 }
 
 /**
- * Observer class that are attached to each observed
- * object. Once attached, the observer converts target
+ * Observer class that is attached to each observed
+ * object. Once attached, the observer converts the target
  * object's property keys into getter/setters that
- * collect dependencies and dispatches updates.
+ * collect dependencies and dispatch updates.
  */
 export class Observer {
   value: any;