[fix](doc) dev: un-glue SET lc_time_names from comment line in MONTHNAME example

[boluor]

The EN dev monthname.md localization example put the explanatory comment and the SET statement on a single -- comment line:

---Control the output language by setting `lc_time_names`SET lc_time_names='zh_CN';
SELECT MONTHNAME('2023-07-13 22:28:18') AS result;
+--------+
| result |
+--------+
| 七月   |
+--------+

Because the entire line is a SQL line-comment, SET lc_time_names='zh_CN'; is commented out. A reader copy-pasting the block runs the SELECT in the default en_US session and gets July, not the documented 七月 — so the localization example is inert and misleading.

Fix: move SET onto its own line (matching the -- comment style used by the other comments in the same block). The documented output values are all correct; only the formatting was broken.

-- Control the output language by setting `lc_time_names`
SET lc_time_names='zh_CN';
SELECT MONTHNAME('2023-07-13 22:28:18') AS result;

Scope: dev EN only. The ZH page (i18n/zh-CN/.../current/.../monthname.md) and the v4.x EN page already have SET on its own line, so they are unaffected — no backport needed.

Verification — run end-to-end on a live master daily build (doris-0.0.0-2e72603618c):

SELECT MONTHNAME('2023-07-13 22:28:18');     -> July
SET lc_time_names='zh_CN';
SELECT MONTHNAME('2023-07-13 22:28:18');     -> 七月
SET lc_time_names='AR_sa';
SELECT MONTHNAME('2023-07-13 22:28:18');     -> يوليو

All three outputs match the documented expected values.

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