Corrected documentation for `lm`: SVD, not QR

I confused the xGELS and xGELSD (note final 'd') functions.  Both solve
linear systems but the former uses the QR decomposition and is used by R's
`lm`, while the latter uses the SVD and is provided by jblas.

A cursory web search doesn't turn up any non-R sources for xGELS (i.e.,
other than [1]). However, at least Intel MKL provides it.

[1] http://www.netlib.org/lapack/double/dgels.f (but *not* in LAPACK)