Always displaying first parameter discription in ParameterInfo tooltip

More information at https://github.com/icsharpcode/NRefactory/pull/390

About this change... This method correctly detect if method has at least 1 parameter set currentParam on 0 from -1(no paramters info). But than wrong variable is used _currentParam(which is still -1) instead of corrected currentParam(0). Resulting in not displaying parameter info.