change branch of tidb-operator

Signed-off-by: liubo02 <liubo02@pingcap.com>

Author: liubog2008

README.md

@@ -19,9 +19,9 @@ Currently, we maintain the following versions for TiDB on Kubernetes documentati
 
 | Branch name | Version description |
 | :--- | :-- |
-| `master` | the latest development version for v1.x |
-| `feature/v2` | the latest development version for v2.x |
+| `main` | the latest development version for v2.x |
 | `release-2.0` | the 2.0 version |
+| `release-1.x` | the latest development version for v1.x |
 | `release-1.6` | the 1.6 version (latest stable) |
 | `release-1.5` | the 1.5 version |
 | `release-1.4` | the 1.4 version |