Excess slash in sed command (after ubuntu before percent)

[lanmaster]

RUN sed -e s%http://archive.ubuntu.com/ubuntu/%mirror://mirrors.ubuntu.com/mirrors.txt% -i /etc/apt/sources.list

do nothing.
But this can make changes in sources.list:
RUN sed -e s%http://archive.ubuntu.com/ubuntu%mirror://mirrors.ubuntu.com/mirrors.txt% -i /etc/apt/sources.list