(DOCSP-35706): Fix broken links

[dacharyc]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-35706

Fix broken links, per the "Broken Links in Docs" report.

C++:

• CRUD/Threading: Fix a broken API reference link to thread_safe_reference.

Flutter:

• Realm Database/CRUD/Read: Fix a broken API reference link to RealmObjects.
• Manage Users/Get an Access Token: Fix a broken link to Dart Timer.periodic().

Reminder Checklist

Before merging your PR, make sure to check a few things.

• [x] Did you tag pages appropriately?
  • genre
  • meta.keywords
  • meta.description
• [x] Describe your PR's changes in the Release Notes section
• [x] Create a Jira ticket for related docs-app-services work, if any

Release Notes

• C++ SDK
  • CRUD/Threading: Fix a broken API reference link.
• Flutter SDK
  • Realm Database/CRUD/Read: Fix a broken API reference link.
  • Manage Users/Get an Access Token: Fix a broken link to a Dart API reference document.

Review Guidelines

REVIEWING.md

[github-actions[bot]]

Readability for Commit Hash: 0f477fe6c969a6ea2c86fce6d6ea542c894e25f6

You can see any previous Readability scores (if they exist) by looking at the comment's history.

Readability scores for changed documents:

• source/sdk/flutter/users/access-token: Grade Level: 6.6, Reading Ease: 62.75
• source/sdk/flutter/crud/read: Grade Level: 8.6, Reading Ease: 57.67
• source/sdk/cpp/crud/threading: Grade Level: 7.8, Reading Ease: 65.12

For Grade Level, aim for 8 or below.

For Reading Ease scores, aim for 60 or above:

Score	Difficulty
90-100	Very Easy
80-89	Easy
70-79	Fairly Easy
60-69	Medium
50-59	Fairly Hard
30-49	Hard
0-29	Very Hard

For help improving readability, try Hemingway App.

[docs-builder-bot]

✨ Staging URL: https://preview-mongodbmongodb.gatsbyjs.io/realm/master/

🪵 Logs

• job log: b4186fe21687f54f1bc899a0827213b0d4d8ddcc