doc: avoid spurious error messages.
doc: avoid spurious error messages.

	* doc/Makefile.am: Prefer "... || true" to "-..." which still
	displays "Error (ignored)": this is not an error.

aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi