Artscroll (in their edition of Rashi, p. 206) translates your quote as "because he caused one more passage of the Torah to be written" and comments
"He caused one more passage of the Torah to be written" does not mean
that there was a change made in the Torah. It is a basic tenet of
Judaism that the Torah can never be changed. Rather, this means that
although Moses' method and criteria for choosing judges could have
been recorded without any mention of Jethro's role, the Torah,
nevertheless, states that Moses' action was based on Jethro's advice.
See also Rashi to Bamidbar 10:29 for a similar comment on Yitro.
Similar examples of passage of the Torah being credited to an individual or group are Bamidbar 25:6 (Pinchas) and Bamidbar 27:5 (daughters of Zelophehad).