Update README.md due to 2.0.0-alpha.2 released (halo-dev#2546)

#### What type of PR is this?

/kind documentation

#### What this PR does / why we need it:

The README.md is out of date due to [2.0.0-alpha.2](https://github.com/halo-dev/halo/releases/tag/v2.0.0-alpha.2) released.

#### Does this PR introduce a user-facing change?

```release-note
None
```

README.md

@@ -35,7 +35,7 @@
 ### Docker
 
 ```bash
-docker run -it -d --name halo-next -p 8090:8090 -v ~/halo-next:/root/halo-next --restart=unless-stopped halohub/halo-dev:2.0.0-alpha.1
+docker run -it -d --name halo-next -p 8090:8090 -v ~/halo-next:/root/halo-next --restart=unless-stopped halohub/halo-dev:2.0.0-alpha.2
 ```
 
 详细部署文档请查阅：<https://docs.halo.run/2.0.0-SNAPSHOT/getting-started/install/docker>