book: english typo (#79)

code/2/2.01.nullptr.cpp

@@ -22,7 +22,7 @@ int main() {
         std::cout << "NULL == nullptr" << std::endl;
 
     foo(0);          // will call foo(int)
-    // foo(NULL);    // doen't compile
+    // foo(NULL);    // doesn't compile
     foo(nullptr);    // will call foo(char*)
     return 0;
 }