fixing 404 link to python doc (#13743)

## DESCRIBE YOUR PR
Fixing a 404
closes #13712

## IS YOUR CHANGE URGENT?  

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to go live.
- [ ] Urgent deadline (GA date, etc.): <!-- ENTER DATE HERE -->
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [ ] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
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Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

Co-authored-by: Shannon Anahata <[email protected]>

Author: sfanahata

includes/custom-measurements-supported-sdks.mdx

@@ -5,7 +5,7 @@
 - [Java (version `6.5.0` or later)](/platforms/java/performance/instrumentation/performance-metrics/)
 - [JavaScript (version `7.0.0` or later)](/platforms/javascript/performance/instrumentation/performance-metrics/)
 - [.NET (version `3.23.0` or later)](/platforms/dotnet/performance/instrumentation/performance-metrics/)
-- [Python (version `1.5.12` or later)](/platforms/python/performance/instrumentation/performance-metrics/)
+- [Python (version `1.5.12` or later)](/platforms/python/tracing/instrumentation/custom-instrumentation/)
 - [React Native (version `4.0.0` or later)](/platforms/react-native/performance/instrumentation/performance-metrics/)
 - [Ruby (version `5.8.0` or later)](/platforms/ruby/performance/instrumentation/performance-metrics/)
 - [Rails (version `5.8.0` or later)](/platforms/ruby/guides/rails/performance/instrumentation/performance-metrics/)