type_object: fix new clippy complaint about length of doc comment

[birkenfeld]

On beta/nightly:

warning: first doc comment paragraph is too long
  --> src/type_object.rs:19:1
   |
19 | / /// `T: PySizedLayout<U>` represents that `T` is not a instance of
20 | | /// [`PyVarObject`](https://docs.python.org/3.8/c-api/structures.html?highlight=pyvarobject#c.PyVarObject).
21 | | /// In addition, that `T` is a concrete representation of `U`.
   | |_
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#too_long_first_doc_paragraph
   = note: `#[warn(clippy::too_long_first_doc_paragraph)]` on by default

[birkenfeld]

I lumped another very minor fix in this...

[davidhewitt]

Thanks!