fix: ai gateway custom provider API URLs

[roerohan]

Summary

Fix the base URL for AI Gateway custom providers

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/ai-gateway/	@abhishekkankani, @palashgo, @thebongy, @kathayl, @mchenco, @cloudflare/pcx-technical-writing