Fix MSVC warning C4819

Replaces a U+00A0 character with regular space, and fixes a typo.

src/json.hpp

@@ -6564,7 +6564,7 @@ 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;