Unbreak doctests

[djc]

#233 regressed a doctest for CRL signing. It turns out CI was using test --all-targets which is documented as omitting doctests:

djc-2021 ci-doctests rcgen $ cargo test -h
Execute all unit and integration tests and build examples of a local package

[..]

Target Selection:
      [..]
      --all-targets       Test all targets (does not include doctests)

Instead, just use cargo test which includes all targets by default.

[cpu]

Good catch, thank you

[est31]

at least according to my 4 year old and probably outdated table there is no single cargo command that tests everything, cargo test misses benches for example. But that's fine as we don't have benches (but we do have doctests so this is an improvement).