Update 05-mdx.mdx . Fix key of the prop (#56883)

Fix key of the prop

Also the same key prop we can find in `next-mdx-remote` [example with getStaticProps](https://github.com/hashicorp/next-mdx-remote#typescript)

Author: dglazkoff

docs/02-app/01-building-your-application/07-configuring/05-mdx.mdx

@@ -185,7 +185,7 @@ export async function getStaticProps() {
   const res = await fetch('https:...')
   const mdxText = await res.text()
   const mdxSource = await serialize(mdxText)
-  return { props: { source: mdxSource } }
+  return { props: { mdxSource } }
 }
 ```
 
@@ -202,7 +202,7 @@ export async function getStaticProps() {
   const res = await fetch('https:...')
   const mdxText = await res.text()
   const mdxSource = await serialize(mdxText)
-  return { props: { source: mdxSource } }
+  return { props: { mdxSource } }
 }
 ```