rename day1.txt -> day01.txt

2020/Program.cs

@@ -50,7 +50,7 @@ namespace AdventOfCode {
       var exampleInput = new List<int>() {
         1721, 979, 366, 299, 675, 1456
       };
-      var testInput = ReadInts(RootDir + "day1.txt");
+      var testInput = ReadInts(RootDir + "day01.txt");
 
       Assert.Throws<ArgumentException>(() => Day1Part1(emptyInput));
       Assert.Throws<ArgumentException>(() => Day1Part1(bogusInput));