weather-forecast: unknown variable declaration syntax

[ErikSchierboom]

The stubs file of the weather-forecast exercise contains the following code:

var (
	CurrentCondition string
	CurrentLocation  string
)

This syntax to declare multiple variables in one block is not known to the student. There are two options to fix this:

1. Introduce this syntax in the basics concept's about.md document as declaring variables is introduced in that concept and it is in the weather-forecast exercise's prerequisites.
2. Rewrite the stub to use separately defined variables

I'm fine with either option.