docs: fix RST error in vm/page_owner.rst

Commit f7df2b1c ("tools/vm/page_owner_sort.c: count and sort by mem")
added a literal text block without the necessary markup, leading to these
warnings in the docs build:

  Documentation/vm/page_owner.rst:92: WARNING: Unexpected indentation.
  Documentation/vm/page_owner.rst:96: WARNING: Unexpected indentation.
  Documentation/vm/page_owner.rst:107: WARNING: Unexpected indentation.

Add the necessary colons and make the build quieter.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>