pdumper-stats now returns s, not ms

* doc/lispref/internals.texi (pdumper-stats):
* src/pdumper.c (pdumper_load): Return seconds, not milliseconds.
Minimize rounding errors in the usual case.
1 job for master in 49 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #600
test-all

00:49:01