doc: Fix the markdown highlighting syntax

[YDX-2147483647]

There should be a new line after [!WARNING], according to community · Discussion #16925.

Before

[!WARNING] uutils is original code and cannot contain any code from GNU or
other implementations. This means that we cannot accept any changes based on
the GNU source code. To make sure that cannot happen, you cannot link to
the GNU source code either.

After

Warning

uutils is original code and cannot contain any code from GNU or
other implementations. This means that we cannot accept any changes based on
the GNU source code. To make sure that cannot happen, you cannot link to
the GNU source code either.

[cakebaker]

Good catch, thanks!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/symlink. tests/tail/symlink is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/truncate. tests/tail/truncate is passing on 'main'. Maybe you have to rebase?