[Text Analytics] Update release date (#11313)

* [Text Analytics] Update release date for 5.1.0-beta.1

* format

sdk/textanalytics/ai-text-analytics/CHANGELOG.md

@@ -1,6 +1,6 @@
 # Release History
 
-## 5.1.0-beta.1 (Unreleased)
+## 5.1.0-beta.1 (2020-09-17)
 
 - We are now targeting the service's v3.1-preview.2 API as the default instead of v3.0.
 - We now have added support for opinion mining feature of Sentiment Analysis. To get this support, pass `includeOpinionMining` as True when calling the `analyzeSentiment` endpoint.
@@ -32,7 +32,6 @@
 - Added `warnings` property to every document response object containing a list of `TextAnalyticsWarning` objects relevant to the corresponding document.
 - Migrated to using the Text Analytics v3.0 (General Availability) service endpoint.
 
-
 ## 1.0.0-preview.4 (2020-04-07)
 
 - Renamed the first parameter of all operation methods from `inputs` to `documents`