docs(mdbook): minor change in the bitwise chiplet

[MaxMustermann2]

Describe your changes

The sentence describing the column $k_0$ had its order reversed, and the
table headers were incorrectly named as $x_i$ and $y_i$ instead of $a_i$
and $b_i$ as described in the introduction.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[Al-Kindi-0]

Thank you @MaxMustermann2 for doing this, it looks great!