chore: minor comment cleanup (#9191)

src/core/observer/dep.js

@@ -49,9 +49,9 @@ export default class Dep {
   }
 }
 
-// the current target watcher being evaluated.
-// this is globally unique because there could be only one
-// watcher being evaluated at any time.
+// The current target watcher being evaluated.
+// This is globally unique because only one watcher
+// can be evaluated at a time.
 Dep.target = null
 const targetStack = []