Unify `array_type` and `array type` usage in the specification

There are a number of places in the spec that talk about arrays and their types, but they do not use a consistent phrase; instead, some use `array_type`, and some use `array type`. As an example, [type inference](https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/expressions.md#12639-exact-inferences) uses `array type`, while [conversions](https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/conversions.md#1028-implicit-reference-conversions) use `array_type`. This makes it difficult to track down all the behavior associated with array types in the specification. It seems like the spec should unify on `array_type`, rather than `array type`, but I don't have a real opinion other than it shouldn't use both.