Corrected documentation for `lm`: SVD, not QR

jblas provides `gelsd` which I confused with another LAPACK function,
xGELS (no final 'd'). Both solve linear systems, but the former uses the
SVD while the latter uses QR decomposition.

Note also that R's `lm` uses the QR decomposition via a custom file
dqrls.f [1] which apparently is from LINPACK.

[1] http://svn.r-project.org/R/trunk/src/appl/dqrls.f