Fix documentation for README.md about certificates generator

[vcerenu]

The README.md file used to create the image used in the certificate generation process has the wrong version tag that is currently used in version 4.8

indexer-certs-creator/README.md file: https://github.com/wazuh/wazuh-docker/blob/f3474a392ee5fb00cfc746144c15aeb9d9e31994/indexer-certs-creator/README.md?plain=1#L1-L9

Certificate Generation Script:

https://github.com/wazuh/wazuh-docker/blob/f3474a392ee5fb00cfc746144c15aeb9d9e31994/single-node/generate-indexer-certs.yml#L1-L10

The README.md file version must be corrected to correctly build the image