Typo: symmetric difference should be the xor, not intersection

[LumioseSil]

Otherwise it is the same as the intersection.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

I'll close and immediately reopen this PR to force CI to pick up the last commits on main. Apologies for the noise.