make: drop the `-c` arg to `install` in the documentation makefile This arg caused `gmake install` on OpenIndiana 2019.10 (illumos) fail with: install: The -c, -f, -n options each require a directory following! install: The -c, -f, -n options each require a directory following! install: The -c, -f, -n options each require a directory following! gmake[1]: *** [Makefile:41: install] Error 2 gmake[1]: Leaving directory '/usr/local/share/mercurial/doc' The workaround is to run `gmake install-bin`. The man page for 10.14 says this is to copy the file and is only for compatability, as it is the default. The CentOS 7 man page says it is ignored. The top level makefile doesn't use this argument at all, so I'm not sure why it's here. Differential Revision: https://phab.mercurial-scm.org/D8439