Fix indentation in usr.bin/diff/pr.c

In commit 6fa5bf0832ef the pr(1) related code in diff was moved around,
but some part of the indentation was messed up, and one line was
duplicated. Remove the duplicated line, and fix up the indentation.

Reviewed by: bapt
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34398

(cherry picked from commit f7d6e97e4bf1fe4c70d5b0ef9fdf649bf07e1c91)