i18n(ja): fix mistranslated letter N in DDL allow-list table

[yahonda]

What is changed, added or deleted? (Required)

Every "N" (meaning "replication not supported") in the DDL allow-list table in ticdc/ticdc-ddl.md was mistranslated as a kanji character meaning "North" — 25 occurrences across 25 rows (DROP INDEX, DROP PRIMARY KEY, CREATE TABLE, DROP TABLE, ADD COLUMN, etc.). The table's own legend directly above it already correctly defines the letter N literally, confirming it was meant to stay as-is in the table body, not be translated.

Also fixed a missing sentence-boundary period that had merged two separate English sentences into one run-on Japanese sentence in the same file.

Which TiDB version(s) do your changes apply to? (Required)

• v8.5 (LTS)
• v8.1 (LTS)
• v7.5 (LTS)
• v7.1 (LTS)
• v6.5 (LTS)
• v6.1 (LTS)

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Add redirect rules
• Change the framework or the code logic of the repository

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