Update rerank example in the README after breaking changes

[rohanshah18]

Problem

Rerank endpoint is updated to accept documents as List<Map<String, Object>> instead of List<Map<String, String>> and parameters now accepts Map<String, Object> instead of Map<String, String>.

Solution

Update the example in README for rerank example

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

NA