[Fleet] Fix readme.md for 0815 (#25822)

* fix readme

* format

Author: zman-ms

specification/containerservice/resource-manager/Microsoft.ContainerService/fleet/readme.md

@@ -27,15 +27,15 @@ These are the global settings for the ContainerServices API.
 
 ``` yaml
 openapi-type: arm
-tag: package-preview-2023-08
+tag: package-2023-08-preview
 ```
 
 
-### Tag: package-preview-2023-08
+### Tag: package-2023-08-preview
 
-These settings apply only when `--tag=package-preview-2023-08` is specified on the command line.
+These settings apply only when `--tag=package-2023-08-preview` is specified on the command line.
 
-```yaml $(tag) == 'package-preview-2023-08'
+``` yaml $(tag) == 'package-2023-08-preview'
 input-file:
   - preview/2023-08-15-preview/fleets.json
 ```