docs: Don't use non-portable cp -d flag unnecessarily (no links are involved)

cp -d is a GNU extension, and only useful for links, which don't seem
to exist in the directories being copied, so just remove said flag.

Signed-off-by: Patrick Welche <prlw1@cam.ac.uk>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

docs/Makefile

@@ -84,9 +84,9 @@ install: all
 	$(INSTALL_DIR) $(DESTDIR)$(DOCDIR)
 
 	$(INSTALL_DIR) $(DESTDIR)$(MANDIR)
-	cp -dR man1 $(DESTDIR)$(MANDIR)
-	cp -dR man5 $(DESTDIR)$(MANDIR)
-	[ ! -d html ] || cp -dR html $(DESTDIR)$(DOCDIR)
+	cp -R man1 $(DESTDIR)$(MANDIR)
+	cp -R man5 $(DESTDIR)$(MANDIR)
+	[ ! -d html ] || cp -R html $(DESTDIR)$(DOCDIR)
 
 html/index.html: $(DOC_HTML) ./gen-html-index INDEX
 	perl -w -- ./gen-html-index -i INDEX html $(DOC_HTML)