[corlib] Fix implementation of Tuple hash generator

The Tuples.cs code generator dies with:

Unhandled Exception:
System.NotImplementedException: The requested feature is not implemented.
  at TupleGen.computeHash (Int32 destVar, Int32 start, Int32 count) [0x00000] in <filename unknown>:0
  at TupleGen.WriteHash (Int32 destVar, Int32 start, Int32 count) [0x00000] in <filename unknown>:0
  at TupleGen.Main () [0x00000] in <filename unknown>:0

This is caused by the incorrect definition of WriteHash(), which
instead of recursively calling itself is calling computeHash().

Thanks to Marek Safar for reporting the issue in
https://github.com/fsharp/fsharp/issues/188#issuecomment-34351850