docs: UML: user_mode_linux_howto_v2 edits

Fix various typos, command syntax, punctuation, capitalization,
and whitespace.

Fixes: 04301bf5

 ("docs: replace the old User Mode Linux HowTo with a new one")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Cc: linux-um@lists.infradead.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Acked-By: anton ivanov <anton.ivanov@cambridgegreys.com>
Link: https://lore.kernel.org/r/20211010064827.3405-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>