Update limits.mdx

[Kiki-Y123]

Number of custom domains per bucket should be 100 instead of 50, ref: https://chat.google.com/room/AAAAJ00jt84/3PkL9Gmq6Pc/3PkL9Gmq6Pc?cls=10

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[cloudflare-docs-bot]

Review

✅ No style-guide issues found in commit 7e38943.

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