refactor snake case

src/io.cpp

@@ -1,14 +1,14 @@
 #include "io.h"
 #include <iostream>
 
-int readNumber()
+int read_number()
 {
     std::cout << "Enter a number to add: ";
     int x{};
     std::cin >> x;
     return x;
 }
-void writeAnswer(int x)
+void write_answer(int x)
 {
     std::cout << "The answer is: " << x << "\n";
 }