Update language-model.md with correct supported lm families

[ozanani]

Hi, there are 2 issues with the current documentation:

1. o1-preview is not available as a model family, it is called o1.
2. gemini-1.5-pro is not available.

See the following screenshot which returns from:

await vscode.lm.selectChatModels({
            vendor: "copilot",
        });

[ntrogh]

@ozanani Thanks for raising this PR. About Gemini not being available, can you check that Gemini preview is enabled for your Copilot subscription (https://github.com/settings/copilot)?

[ozanani]

@ntrogh it does not appear in my settings.

[ntrogh]

@ozanani I seem to have mixed up with the Claude model setting... Gemini support was announced at GitHub Universe, but agree it looks it's not available in the API yet.