fix typo in Juniper TOTP form name

fix typo in Juniper TOTP form name

There's a thread from November 2017 in which Kevin Cernekee and Andy
Wang discuss a Juniper VPN where TOTP fails to work:
http://lists.infradead.org/pipermail/openconnect-devel/2017-November/004569.html

… and in that thread both "ftmTotpToken" and "frmTotpToken" are
mentioned. It appears that the reason for the misbehavior discussed in
the November 2017 is because of the (overlooked) typo in the code.

I'm not sure how the "ftm" version got committed in the first
place. ¯\_(ツ)_/¯. I did a bit of archeological digging, and I can't
find the original patch (from November 29, 2016) anywhere on the
mailing list, but it's obviously in the Git history:
http://git.infradead.org/users/dwmw2/openconnect.git/commitdiff/1ff34cb9689fbaf57decac537df1e32e799bb9c7

Signed-off-by: Daniel Lenski <dlenski@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>