[fix](doc) v2.1: fix point-query backtick and SECOND() expected output

[boluor]

Backport of #3853 + #3854 to version-2.1 (both are EN-only typos/expected-value fixes; ZH already correct on these pages).

Problems

1. high-concurrent-point-query.md — misplaced backtick in DISTRIBUTED BY HASH(k1)`` (should be HASH(k1`)`). The malformed column reference makes the parser reject the whole `CREATE TABLE`: `missing ')' at 'BUCKETS'`.
2. second.md — the example select second('2018-12-31 23:59:59') is documented with value 0 and a mismatched header cast('2018-12-30' ...); the correct result is 59.

Cluster verification (Doris 2.1.11-rc01)

-- point-query CREATE TABLE with HASH(`k1`)  -> Query OK; table created
-- second:
mysql> select second('2018-12-31 23:59:59');
+-------------------------------+
| second('2018-12-31 23:59:59') |
+-------------------------------+
|                            59 |
+-------------------------------+

🤖 Generated with Claude Code