Changing to typo introduced

src/json.hpp

@@ -6578,7 +6578,7 @@ class serializer
                         // check that the additional bytes are present
                         assert(i + bytes < s.size());
 
-                        // to use \uxxxx escaping, we first need to caluclate
+                        // to use \uxxxx escaping, we first need to calculate
                         // the codepoint from the UTF-8 bytes
                         int codepoint = 0;