fix: update method name in GeminiClient

[sugatoray]

• Change GeminiClient method name from _initialize_vartexai to _initialize_vertexai.
• Why? vartexai is a typo. It should be vertextai instead. This PR fixes that.

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. NA
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[BeibinLi]

@sugatoray Thanks for fixing the typo!~

[sugatoray]

@BeibinLi I see the tests are failing for a different reason apparently. Can you please go ahead and merge the PR if it looks okay to you?

[Hk669]

@BeibinLi I see the tests are failing for a different reason apparently. Can you please go ahead and merge the PR if it looks okay to you?

once the LLamaIndexAgent tests are fixed we'll merge this PR. thanks