i18n(ja): remove stray quotes and trapped particles in monitor-alert-pagerduty.md

[yahonda]

What is changed, added or deleted? (Required)

Fixes several markup defects in tidb-cloud/monitor-alert-pagerduty.md's 3 near-identical plan sections (Dedicated/Essential/Premium), found via a user spot-check of the "Subscriber Type" line:

• Particles (の/を) trapped inside a bold span's closing ** instead of sitting outside it, e.g. **購読者タイプの**ドロップダウンリストから**「PagerDuty」を**選択してください。
• Japanese kagi-kakko quotes (「」) around labels that EN's own doc never quotes: Subscriber Type, PagerDuty, Name, Integration Key, Test Connection, Save, Subscribe, Project Settings, Alert Subscription, Add Subscriber, Unsubscribe — all confirmed plain **Label** in the EN source at every corresponding position.

Example:

- **購読者タイプの**ドロップダウンリストから**「PagerDuty」を**選択してください。
+ **購読者タイプ**のドロップダウンリストから**PagerDuty**を選択してください。

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• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

What is the related PR or file link(s)?

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• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[yahonda]

Folding this into #23552 instead (same defect class).

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