Make "forecasts" singular

[guardrex]

A forecast includes multiple events by definition, so change "forecasts" in code examples to "forecast." One wouldn't ask, "Can I have next week's weather forecasts?" 🤮

Mirror changes in the project templates.

Mirrored for Blazor samples by dotnet/blazor-samples#542.

[guardrex]

It's an array of daily forecasts, so it doesn't compose well with the current naming.