[Rd] Typo in help(optim) (PR#6847)

[stephen at anc.ed.ac.uk]

I think there is a small typo in "help(optim)":

     Method '"L-BFGS-B"' is that of Byrd _et. al._ (1994) ...

In the reference list, there is no 1994 reference, but there is
Byrd et al. (1995).

Stephen