fix(docs): Change 'a Action' to 'an Action' in useOptimistic.md

This fixes a grammar error where 'a Action' should be 'an Action' since
'Action' starts with a vowel sound.

Related to #6713 - Capitalize React concepts in docs

Author: MorikawaSouma

src/content/reference/react/useOptimistic.md

@@ -83,7 +83,7 @@ function handleClick() {
 
 #### How optimistic state works {/*how-optimistic-state-works*/}
 
-`useOptimistic` lets you show a temporary value while a Action is in progress:
+`useOptimistic` lets you show a temporary value while an Action is in progress:
 
 ```js
 const [value, setValue] = useState('a');