doc/prerst2man.py: Fix 'os.system' -> 'system' typo

doc/prerst2man.py: Fix 'os.system' -> 'system' typo

Avoid:

  $ make HAVE_SPHINX=0 HAVE_RST2MAN=1 build-man
  python ./doc/prerst2man.py ./doc doc/_build/man
  Traceback (most recent call last):
    File "./doc/prerst2man.py", line 65, in <module>
      os.system('set -x; rst2man {0} {1}/{2}.{3}'
  NameError: name 'os' is not defined
  make: *** [doc/_build/man/man1/notmuch.1] Error 1

by using system directly.  We don't need the 'os.' namespacing,
because the function was imported with:

  from os import makedirs, system