Update the name of the dart_community discord (#5120)

The Dart community Discord has renamed to dart_community to match Dart's
package/file naming scheme :)

Author: parlough

src/community/index.md

@@ -33,12 +33,12 @@ Get answers and connect with Dart developers.
 [StackOverflow](https://stackoverflow.com/tags/dart)
 : The best place for how-to questions.
 
+[The dart_community Discord](https://discord.gg/Qt6DgfAWWx)
+: Chat with and get help from other Dart developers.
+
 [Dart on Reddit](https://www.reddit.com/r/dartlang)
 : The subreddit for all things related to Dart.
 
-[The Unofficial Dart Community Discord](https://discord.gg/Qt6DgfAWWx)
-: Chat with and get help from other Dart developers.
-
 #### Google Groups
 
 [General discussions]({{page.group}}/d/forum/misc)