fixes info about installation into a specific directory

Makefile

@@ -175,8 +175,8 @@ ifeq ($(FRAMAC_INSTALLDIR),"")
 	dune install
 else
 	dune install --prefix ${FRAMAC_INSTALLDIR}
-	echo 'DO NOT FORGET TO EXPAND YOUR OCAMLPATH VARIABLE:'
-	echo '  export OCAMLPATH="${FRAMAC_INSTALLDIR}:$$OCAMLPATH"'
+	@echo 'DO NOT FORGET TO EXPAND YOUR OCAMLPATH VARIABLE:'
+	@echo '  export OCAMLPATH="${FRAMAC_INSTALLDIR}/lib:$$OCAMLPATH"'
 endif
 
 uninstall: