Update broken API link on Index basics page

[KOTungseth]

On the Index basics page, the frozen link to the API docs results in a 404.

[lcawl]

FYI That API was removed in elastic/elasticsearch#119227 and there's rationale for the removal in https://www.elastic.co/guide/en/elasticsearch/reference/8.19/unfreeze-index-api.html

[bmorelli25]

sorry if it's just removing the link then this is easy. if it's not, sending to admin docs board.