fix: update oauth2 java library link

[YassineHaouzane]

Related Issue or Design Document

The current url of spring-security in the using-oauth2 page leads to a 404.
I have updated with the same link that is used on this page https://oauth.net/code/

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

[jonas-jonas]

Awesome. Nice catch. Thanks!