docs(read): point the head/tail rewrite docs at --head-lines

The pipeline diagram, the discover walkthrough and the `rtk read` option
table still described the `--max-lines` mapping this branch replaced, and
the repo's own suggest hook still proposed `--max-lines N` for `head -N`
— the mapping this branch exists to correct.

Drop the `head -` disjunct in the rewrite guard: every string with that
prefix also has the `head ` prefix that sits next to it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>