OAuth Configuration Documentation: "client_secret" is "client_secrets" - adapt documentation

**Expected Behavior**

The documentation (OAuth configuration) matches the tool (version 4.2.0).

**Actual Behavior**

The documentation for OAuth does not match the config keys in 4.2.0:

The documentation at https://github.com/adobe/aio-cli-plugin-cloudmanager?tab=readme-ov-file#setup-for-oauth-integration mentions that the OAuth config has a key "client_secret", but the field is now called "client_secrets" (for OAuth only) and has become a list property, see e.g. here: https://github.com/adobe/aio-cli-plugin-cloudmanager/blob/main/e2e/e2e.js#L72

**Reproduction Scenario, Platform, and Version**

Version 4.2.0

**Steps to Reproduce**

Setup aio cli-cloudmanager using OAuth according to documentation https://github.com/adobe/aio-cli-plugin-cloudmanager?tab=readme-ov-file#setup-for-oauth-integration
You will get an HTTP 400 (parameter client_secrets missing)

Start with the OAuth configuration section in the repository README and compare its documented keys with e2e/e2e.js around line 72 for version 4.2.0. Update the documentation so the OAuth configuration matches the tool, then verify the setup steps no longer refer to the missing parameter.

Written by the indexing model from the issue text.