pdftops: fix wording in processcolorformat-related error messages

Status Job ID Name Coverage
  Build
passed #4714906
build

00:05:22

passed #4714911
build_android

00:00:41

passed #4714907
build_clang_libcpp

00:05:09

passed #4714910
build_clazy

00:03:04

passed #4714909
build_mingw64_fedora

00:03:13

passed #4714908
build_ubuntu_16_04

00:02:36

passed #4714905
clang_format

00:01:09