lang/ocaml: unbreak with DOCS=ON after 3fc4fad2 (+)

Commit 3fc4fad2 drops extra '/' from from _DISTDIR
Meanwhile ocaml Makefile stay relies on this extra '/' presence which caused wromg path generation for doc distfile:

install: /portdistfilesocaml-4.14-refman.pdf: No such file or directory

Reported by:	bulk -t
Approved by:	portmgr blanket