Seems to be that docs/man/ctr.1.md and docs/man/containerd.1.md were
removed in #3637 and were not updated correctly in the Makefile, leading
to build failures like:
+ make man
make: *** No rule to make target `man/ctr.1', needed by `man'. Stop.
Changes the gen-manpages command to be specific on which manpages are to
be generated.
Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>