Fix building of hcheck man page

The code used a space, not a tab, in Makefile.am, which means it
actually broke the variable declaring the man pages.

Furthermore, we add some blank lines in the man page, otherwise the
paragraphs will be run-in together.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>