[CssSelector] Use the correct cssselect library name in docblocks.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #11894
| License       | MIT
| Doc PR        | -

As suggested in #11894, the original python library is called [cssselect](https://github.com/SimonSapin/cssselect), not cssselector.

I also updated the README, as the cssselect was extracted as an independent library and it's no longer part of lxml.