configure: Fix help string for --with-background-end-color-stop

Looks like it was copied from --with-background-start-color-stop and
never updated to reflect the option it is describing.
2 jobs for fix-configure-help-string in 2 minutes and 53 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed debian-stable-x86_64 #15696663

00:02:25

passed fedora-x86_64 #15696662

00:02:53