example of partition table in tidb_http_api.md is weird

[ekexium]

Bug Report

*Hint: On a partitioned table, use the `table(partition)` pattern as the table name, `test(p1)` for example:*

```shell
$curl http://127.0.0.1:10080/mvcc/index/test(p1)/t1/idx/1\?a\=A
```

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3. What did you see instead (Required)

4. What is your TiDB version? (Required)