Trivial documentation fix regarding Obj-C ARC objc_arc_weak_reference_unavailable

Fixed incorrect docs that referred to:
  objc_arc_weak_unavailable
when it should be:
  objc_arc_weak_reference_unavailable

Patch by: Sean McBride!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281227 91177308-0d34-0410-b5e6-96231b3b80d8