Update process filter example (Bug#35044)

* doc/lispref/processes.texi (Filter Functions): Use
insert-before-markers in the "ordinary" filter example, like
internal-default-process-filter does.
1 job for emacs-26 in 26 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1597
test

00:26:27