Fix man page installation
Fix man page installation

The trailing slash in `${CMAKE_SOURCE_DIR}/man/` is important, as we do not want to install the man pages in `${CMAKE_INSTALL_PREFIX}/man/man1/man`, but rather just `${CMAKE_INSTALL_PREFIX}/man/man1`. (I encountered this problem when building on macOS.)

Per the [documentation](https://cmake.org/cmake/help/latest/command/install.html):

> The DIRECTORY form installs contents of one or more directories to a given destination. The directory structure is copied verbatim to the destination. The last component of each directory name is appended to the destination directory but a trailing slash may be used to avoid this because it leaves the last component empty.
vcflib/vcflib,ekg/vcflib,ekg/vcflib,vcflib/vcflib,ekg/vcflib,vcflib/vcflib,ekg/vcflib,ekg/vcflib,vcflib/vcflib,vcflib/vcflib,vcflib/vcflib,vcflib/vcflib,vcflib/vcflib