Fix broken markup, a blank quiz answer, and five misleading statements

[wdecoster]

Follow-up to #1, from reading the English course end to end. Two commits, kept separate so the mechanical half can be taken on its own if you prefer.

Commit 1 — broken markup and a blank quiz answer

Six mechanical defects, all visible on the published site today:

lesson	problem
rm	asks "How do you remove a file called myfile?" and the answer panel is blank — the ## Quiz Answer section is empty
regex	an unclosed <pre> swallows the Quiz Question and Answer into the code block; src/main.py then finds no <h3>Quiz Question</h3> to split on, so the quiz panel renders as a stray > and the page HTML comes out malformed (</pre></div\n>)
umask	<ol> opened twice, never closed
bootloader	missing </li>
systemd overview	</l> typo
modifying permissions	chmod ug+w has no filename — the example as printed just errors

After this, no lesson in the course has unbalanced <ol>/<ul>/<li>/<pre>/<b>/<i> tags, and no lesson asks a quiz question without answering it.

Commit 2 — five statements that mislead beginners

• help: the --help convention is introduced with echo --help. echo is a shell built-in that doesn't accept it, so the command prints the literal text --help. Switched to ls --help and made the built-in vs program distinction explicit — which is what the lesson is really about.
• umask: says a umask of 022 means "all user access", implying new files come out rwx. Files start from 666, not 777, so 022 gives 644 for files and 755 for directories. The exercise currently asks readers to predict permissions they then won't see. Added the starting points and the arithmetic.
• rm: rmdir was presented as just another way to remove a directory; it only works on empty ones, which is the entire reason to prefer it over rm -r.
• cp: "You can use wildcards in every command" reads as though each command implements globbing. The shell expands them before the command runs.
• yum and apt: install/remove/update shown without sudo, so a normal user copying them hits a permission error. sudo is taught three sections earlier in Users and Groups.

How the generated files were produced

Not hand-edited. Lesson HTML comes from src/convert.py's converter; the docs/ pages are rewritten with src/main.py's split rules and templates/lesson.html's block layout. Both were checked against lessons this PR does not touch and reproduce them byte-for-byte, so each diff contains only the change described above.