Fix broken link in error codes doc and support doc (#6196) (#6199)

error-codes.md

@@ -9,7 +9,7 @@ This document describes the problems encountered during the use of TiDB and prov
 
 ## Error codes
 
-TiDB is compatible with the error codes in MySQL, and in most cases returns the same error code as MySQL. For a list of error codes for MySQL, see [Server Error Message Reference](https://dev.mysql.com/doc/refman/5.7/en/server-error-reference.html). In addition, TiDB has the following unique error codes:
+TiDB is compatible with the error codes in MySQL, and in most cases returns the same error code as MySQL. For a list of error codes for MySQL, see [MySQL 5.7 Error Message Reference](https://dev.mysql.com/doc/mysql-errors/5.7/en/). In addition, TiDB has the following unique error codes:
 
 > **Note:**
 >

support.md

@@ -7,7 +7,7 @@ summary: Find support resources for your TiDB installation.
 
 You can reach out to the community members via any one of the following ways:
 
-+ Slack Channel: [https://pingcap.com/tidbslack](https://pingcap.com/tidbslack)
++ Slack Channel: [https://slack.tidb.io/](https://slack.tidb.io/invite?team=tidb-community&channel=everyone&ref=pingcap)
 + Google Groups: [https://groups.google.com/forum/#!forum/tidb-user](https://groups.google.com/forum/#!forum/tidb-user)
 + Stack Overflow: [https://stackoverflow.com/questions/tagged/tidb](https://stackoverflow.com/questions/tagged/tidb)
 + Twitter: [https://twitter.com/PingCAP](https://twitter.com/PingCAP)