Fix typo in readme

[ole]

The readme mistakenly refers to "Swift Numerics" in one place. Looks like a copy/paste oversight introduced in #167.

Checklist

• [ ] (n/a) I've added at least one test that validates that my change is working, if appropriate
• [ ] (n/a) I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary