[KYUUBI apache#4627] [Docs] Fix a typo in rest_api.md.

### _Why are the changes needed?_

`GET /batches/{batchId}` should be `GET /batches/${batchId}`

Affected versions 1.6/1.7/master

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [ ] [Run test](https://kyuubi.readthedocs.io/en/master/develop_tools/testing.html#running-tests) locally before make a pull request

Closes apache#4627 from merrily01/master-typo.

Closes apache#4627

ac386f6 [maruilei] [Docs] Fix a typo in rest_api.md.

Authored-by: maruilei <maruilei@58.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>

docs/client/rest/rest_api.md

@@ -397,7 +397,7 @@ curl --location --request POST 'http://localhost:10099/api/v1/batches' \
 
 The created [Batch](#batch) object.
 
-### GET /batches/{batchId}
+### GET /batches/${batchId}
 
 Returns the batch information.