[Rd] typo in R-exts

[Torsten Hothorn]

Hi,

the definition of `fsign' in R-exts currently is

double fsign (double x, double y) Function
  Performs "transfer of sign" and is defined as |x| * sign(x)

but should read

double fsign (double x, double y) Function
  Performs "transfer of sign" and is defined as |x| * sign(y)
                                                      ^^^^^^^

Best,

Torsten