doc: sea.getRawAsset(key) always returns an ArrayBuffer

PR-URL: #56206
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/single-executable-applications.md

@@ -352,7 +352,7 @@ writes to the returned array buffer is likely to result in a crash.
 
 * `key`  {string} the key for the asset in the dictionary specified by the
   `assets` field in the single-executable application configuration.
-* Returns: {string|ArrayBuffer}
+* Returns: {ArrayBuffer}
 
 ### `require(id)` in the injected main script is not file based