doc: fix grammar in explanation of duneVersion in buildDunePackage

- It uses the `fetchFromGitHub` fetcher to get its source.

- It also accept `duneVersion` parameter (valid value are `"1"`, `"2"`, and

  `"3"`). The recommended practice it to set only if you don't want the default

- It also accepts a `duneVersion` parameter (valid values are `"1"`, `"2"`, and

  `"3"`). The recommended practice is to set it only if you don't want the default

  value and/or it depends on something else like package version. You might see

  a not-supported argument `useDune2`. The behavior was `useDune2 = true;` =>

  `duneVersion = "2";` and `useDune2 = false;` => `duneVersion = "1";`. It was

  used at the time when dune3 didn't existed.

  used at the time when dune3 didn't exist.

- It sets the optional `doCheck` attribute such that tests will be run with

  `dune runtest -p angstrom` after the build (`dune build -p angstrom`) is