[.Net] update GeminiChatAgent.cs

[eltociear]

Why are these changes needed?

multipe -> multiple

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[LittleLittleCloud]

@eltociear Hey, thanks for the PR, we are going through a major refactor on a new AutoGen architecture for the main branch currently so we may have to postpone merging this PR until then. You can find out more information for the new architecture under #3601.

[eltociear]

@LittleLittleCloud Thank you for contacting me! Noted!

[rysweet]

@LittleLittleCloud - can you decide whether this work should come forward and be adapted to 0.4?

[rysweet]

This PR is against AutoGen 0.2. AutoGen 0.2 has been moved to the 0.2 branch. Please rebase your PR on the 0.2 branch or update it to work with the new AutoGen 0.4 that is now in main.