Merge pull request #2032 from ForrestGumb/patch-3

Update rest-text-to-speech.md

Author: prmerger-automator[bot]

articles/ai-services/speech-service/rest-text-to-speech.md

@@ -258,6 +258,7 @@ The HTTP status code for each response indicates success or common errors:
 | 415 | Unsupported media type | It's possible that the wrong `Content-Type` value was provided. `Content-Type` should be set to `application/ssml+xml`. |
 | 429 | Too many requests | You exceeded the quota or rate of requests allowed for your resource. |
 | 502 | Bad gateway    | There's a network or server-side problem. This status might also indicate invalid headers. |
+| 503 | Service Unavailable    | There's a server-side problem for various reasons. |
 
 If the HTTP status is `200 OK`, the body of the response contains an audio file in the requested format. This file can be played as it's transferred, saved to a buffer, or saved to a file.