docs: fix duplicate article and incorrect article usage

- Remove duplicate 'a' in resumeAndPrerenderToNodeStream.md ('a a' -> 'a')
- Fix article 'a Action' -> 'an Action' in useOptimistic.md

Author: seojcarlos

src/content/reference/react-dom/static/resumeAndPrerenderToNodeStream.md

@@ -4,7 +4,7 @@ title: resumeAndPrerenderToNodeStream
 
 <Intro>
 
-`resumeAndPrerenderToNodeStream` continues a prerendered React tree to a static HTML string using a a [Node.js Stream.](https://nodejs.org/api/stream.html).
+`resumeAndPrerenderToNodeStream` continues a prerendered React tree to a static HTML string using a [Node.js Stream.](https://nodejs.org/api/stream.html).
 
 ```js
 const {prelude, postponed} = await resumeAndPrerenderToNodeStream(reactNode, postponedState, options?)

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');