Make type arguments in (=) declaration implicit (for docs)

Just a tiny cleanup to make the display of the `(=)` function more consistent with what should be displayed; the types should be *implicit* arguments.

Before:
```
Idris> :t (=)
= : ({A0} : Type) -> ({B0} : Type) -> A -> B -> Type
```

After:
```
Idris> :t (=)
= : A -> B -> Type
```