Fixed range check for grayscale output

src/iomanip.cpp

@@ -1,7 +1,7 @@
 #include "iomanip.hpp"
 
 static std::ostream& gray(std::ostream& out, int val, int code) {
-    if (val < 0 || val > 24) {
+    if (val < 0 || val > 23) {
         throw std::invalid_argument("gray value out of range");
     }
     out << "\x1b[" << code << ";5;" << 232 + val << "m";