refactor: reset variable names that don't fit naming

Author: oereo

src/main/java/calculator/Calculator.java

@@ -15,17 +15,17 @@ public void calculation(){
     public void OperatorSetting(ArithmeticExpressionStack arithmeticExpressionStack) {
         ErrorException exception = new ErrorException();
         String firstString = arithmeticExpressionStack.pop();
-        int firstNumber = exception.NumericalError(firstString);
+        int cumulativeResult = exception.NumericalError(firstString);
 
         for(int i = 0; i<(arithmeticExpressionStack.getStackSize())/2;i++){
             String operator = arithmeticExpressionStack.pop();
             String secondString = arithmeticExpressionStack.pop();
             int secondNumber = exception.NumericalError(secondString);
-            firstNumber = chooseOperatorAndCalculate(firstNumber, operator, secondNumber);
+            cumulativeResult = chooseOperatorAndCalculate(cumulativeResult, operator, secondNumber);
         }
 
-        if(firstNumber != 0){
-            message.calculationResult(firstNumber);
+        if(cumulativeResult != 0){
+            message.calculationResult(cumulativeResult);
         }
         init();
     }