Small units typo in gsw_Nsquared

[richardsc]

Went looking for this based on TEOS-10/GSW-Python#91 and found that we have the wrong unit too (but oddly, it does have the "^" for the exponent).

GSW-R/R/gsw.R

Line 3049 in 1846e10

#' @return list containing N2 [ 1/s^ ] and mid-point pressure p_mid [ dbar ]

[dankelley]

Good catch!! I've fixed this in "develop" commit 3962853