Fix glancing blows formula for classic

In classic a difference in skill points should reward an extra 2% chance of getting a glancing blow.

source: http://www.wowwiki.com/Glancing_blow#Glancing_Blow_chance

This commit [c041d59](http://github.com/mangos-zero/server-old/commit/c041d59) is one of those pending backports from old mangos-zero concatenated in: https://github.com/cmangos/issues/wiki/classic_backporting-todo-zero-commits

I tested it and this is my report:
> See source: http://www.wowwiki.com/Glancing_blow#Glancing_Blow_chance