manpages: Fix small errors in documentation

Mostly typos, except for the output of "gnt-instance migrate" in an
example, which has been updated to the current version

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>