Update the header of the RRNom output envelope files

The current header shows:

Frequency Median Mag Phase (Rad) -1 Sigma Mag -1 Sigma Phase +1 Sigma Mag +1 Sigma Phase

That's not correct. In the new version, the +/-1 sigma should be changed to the 84th and 16th percentiles to avoid confusion.

@evan-goetz I'm not sure if this is the right place to report this issue. Please feel free to move it to where it fits.