docs: Replace deprecated --preserve-merges with --rebase-merges

[choznerol]

Fixes #217

I'm not 100% sure about recommending --rebase-merges, or if the default behavior (dropping merge commits) would be just fine.

We may want git rebase upstream/master --rebase-merges because there is probably a reason we recommended --preserve-merges in the first place. According to the documentation, --rebase-merges is most similar to --preserve-merges:

The `--rebase-merges` mode is similar in spirit to the deprecated `--preserve-merges` but works with interactive rebases, where commits can be reordered, inserted, and dropped at will.

On the other hand, if we're unclear why we recommended --preserve-merges in the first place (considering #217 might have been an issue since 2021), maybe the simpler git rebase upstream/master is just better.

Disclaimer: I never knew about --preserve-merges or --rebase-merges before #217.