Updated the header file to calculator.h

src/main/c/testForNumber.c

@@ -3,7 +3,7 @@
 #include <stdlib.h>
 #include <math.h>
 
-#include "taschenrechner.h"
+#include "calculator.h"
 
 double testForNumber() {
     double num;
@@ -27,4 +27,4 @@ double testForNumber() {
     } while (!isspace(*notANumber) && *notANumber != 0);
 
     return num;
-}
+}