[DOC] Update default value for engine in k-NN index

[VijayanB]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.
In 2.18, we will be updating default engine from nmslib to fiass. We would like to update the same in documentation
https://opensearch.org/docs/latest/search-plugins/knn/knn-index/#method-definitions

*Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.
2.18

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
opensearch-project/k-NN#2163
PR is raised to make that change in code here opensearch-project/k-NN#2221