fix(v2): correct typo in metas generated for Twitter cards (#3900)

The Open Graph protocol format is the following:

```
<meta property="og:image" content="https://example.com/image.png />
```

Twitter metas, however, are structured in a different format:

```
<meta name="twitter:image" content="https://example.com/image.png />
```

They are similar but not identical, I think this led to confusion and
the typo we have in the codebase.

packages/docusaurus-theme-bootstrap/src/theme/DebugLayout/index.tsx

@@ -58,7 +58,7 @@ function Layout(props: Props): JSX.Element {
           <meta name="keywords" content={keywords.join(',')} />
         )}
         {metaImage && <meta property="og:image" content={metaImageUrl} />}
-        {metaImage && <meta property="twitter:image" content={metaImageUrl} />}
+        {metaImage && <meta name="twitter:image" content={metaImageUrl} />}
         {metaImage && (
           <meta name="twitter:image:alt" content={`Image for ${metaTitle}`} />
         )}