Fix documentation link to submodule that doesn't work on GitHub

Fixes #10458

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Author: gilles-peskine-arm

docs/4.0-migration-guide.md

@@ -11,7 +11,7 @@ The changes are detailed below. Here is a summary of the main points:
 - The cryptography API is now mostly the PSA API: most legacy cryptography APIs have been removed. This has led to adaptations in some X.509 and TLS APIs, notably because the library always uses the PSA random generator.
 - Various deprecated or minor functionality has been removed.
 
-Please consult the [TF-PSA-Crypto migration guide](../tf-psa-crypto/docs/1.0-migration-guide.md) for all information related to the crytography part of the library.
+Please consult the [TF-PSA-Crypto migration guide](https://github.com/Mbed-TLS/TF-PSA-Crypto/blob/development/docs/1.0-migration-guide.md) for all information related to the crytography part of the library.
 
 ## CMake as the only build system
 Mbed TLS now uses CMake exclusively to configure and drive its build process.