add space to include

src/calc.cpp

@@ -2,7 +2,7 @@
 
 #include <stack>
 #include <stdexcept>
-#include<cmath>
+#include <cmath>
 
 int plus(    int a, int b) { return a + b; }
 int minus(   int a, int b) { return a - b; }