Rephrase dumb to silly (#1525)

Replace dumb with silly, since dumb means mute or unable to speak. What the document means here is that this function call may seem to be a bit extra and non obvious at first glance. The whimsical word "silly" is a nice stand in for this.

Author: Brian Anderson

Diff for: docs/guide/actions.md

@@ -47,7 +47,7 @@ Actions are triggered with the `store.dispatch` method:
 store.dispatch('increment')
 ```
 
-This may look dumb at first sight: if we want to increment the count, why don't we just call `store.commit('increment')` directly? Remember that **mutations have to be synchronous**? Actions don't. We can perform **asynchronous** operations inside an action:
+This may look silly at first sight: if we want to increment the count, why don't we just call `store.commit('increment')` directly? Remember that **mutations have to be synchronous**? Actions don't. We can perform **asynchronous** operations inside an action:
 
 ``` js
 actions: {