fix unicode typo
[yang] fix unicode typo

Summary:
there are two commas, but the first comma is not the right comma, but the unicode, that means, that
the atom none is the none, (which may be lead to a bug, that nobody find before..I do not know exactly).

It was found by compilation in R17 by Vladimir, because of the update from unicode to the utf8.

Test Plan: check compilation of your yang-based projects

Reviewers: aschultz, vtarasen, akuleshov, hsychla

Reviewed By: vtarasen

CC: mkraemer, aschultz

Differential Revision: https://review.tpip.net/D1235