Kleene operator section does not specify minimum Rust version

[NPN]

The section "At most one repetition" does not specify a minimum Rust version. On the latest stable (rustc 1.31.0 (abe02cefd 2018-12-04)), the operator is still unstable:

error[E0658]: using the `?` macro Kleene operator for "at most one" repetition is unstable (see issue #48075)                                  

A comment on the PR adding the section suggests that the operator will actually be added in 1.32. If so, a badge should be added to the section.