Restore mlr manpage to distro file

Between 5.9 and 5.10, in the move of docs from https://johnkerl.org/miller/doc to https://miller.readthedocs.io/, I inadvertently made a change which kept the Miller manpage (man mlr) from being included in the distribution file.

The sole purpose of this release is to fix that.

If your way to access Miller versions is by downloading pre-built executables from the release page, or by building from source, this release doesn't do much for you. It's most useful for OS-specfic distro-build systems, so that man mlr will again work correctly.