correction to dihedral_class2.cpp

Greetings

Lili Zhang noticed a misleading comment located near line 233 of
dihedral_class2.cpp:

    // addition by Andrew Jewett, Jan 2013
    // adjust the sign of phi if necessary for negative input angles
    // n123 = vb2 x vb1

It should be
    // n123 = vb1 x vb2

Other than the comment, which is confusing, the code still appears to
behave correctly. (I tested the Jan 20th 2014 version of the code
yesterday.)

Cheers
Andrew

updated the comment in the src file

Steve