[minor] Makefile.menhir

This fixes a problematic issue to a wrong Makefile.menhir that
was supposed to have been fixed by f10736b9e6, but actually was not.

Apologies for the flawed commits and for the noise.

(This is the sort of horror sorry when the `mv`, which intended to move
temporary Menhir-produced files to `boot/`, was wrongly placed (by myself)
on the call to copy the runtime files from Menhir's installation directory
into `boot/`. As a result, running 'promote-menhir' would break your Menhir
install...)

Makefile.menhir

@@ -84,7 +84,7 @@ promote-menhir: parsing/parser.mly
 .PHONY: import-menhirLib
 import-menhirLib:
 	mkdir -p boot/menhir
-	mv \
+	cp \
            $(addprefix `$(MENHIR) --suggest-menhirLib`/menhirLib.,ml mli) \
            boot/menhir