html: case sensitive file systems require matching require args.

* ldoc/html.lua: Make sure to `require 'pl.OrderedMap'` (with a
capital 'O') to match the filename installed by penlight.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Merge pull request #151 from gvvaughan/pull-request/case-sensitive-filesystems

html: case sensitive file systems require matching require args.