[fix](doc) v2.1: replace stray ${tableName_21} placeholder in to-quantile-state

[boluor]

Backport of #3852 to version-2.1.

Problem

The ZH to-quantile-state.md CREATE TABLE example carries the unsubstituted doc-template variable ${tableName_21}, while the INSERT/SELECT that follow reference the literal quantile_state_agg_test. On a real cluster the CREATE makes a table literally named ${tableName_21}, so every later statement fails with table-not-found.

Fix

Use the literal name quantile_state_agg_test in the CREATE TABLE.

Cluster verification (Doris 2.1.11-rc01, fresh single-node cluster)

SELECT dt, id, quantile_percent(quantile_union(price), 0)
  FROM quantile_state_agg_test GROUP BY dt, id ORDER BY dt, id;
+----------+------+--------------------------------------------+
| dt       | id   | quantile_percent(quantile_union(price), 0) |
+----------+------+--------------------------------------------+
| 20220201 |    0 |                                          1 |
| 20220201 |    1 |                                         -1 |
+----------+------+--------------------------------------------+

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