[Rd] Typo in ?options for warning.length (PR#10536)

[phgrosjean at sciviews.org]

In ?options, one reads warnings.length, but it should be warning.length.

Version:
  platform = i386-apple-darwin8.10.1
  arch = i386
  os = darwin8.10.1
  system = i386, darwin8.10.1
  status =
  major = 2
  minor = 6.1
  year = 2007
  month = 11
  day = 26
  svn rev = 43537
  language = R
  version.string = R version 2.6.1 (2007-11-26)