Update parse_exceptions.md

Referring to https://github.com/nlohmann/json/blob/develop/doc/examples/parse_error.cpp  and https://json.nlohmann.me/home/exceptions/
(and also based on testing), the catch command should reference `catch (json::parse_error& ex)` and not `catch (json::exception::parse_error& ex)`

doc/mkdocs/docs/features/parsing/parse_exceptions.md

@@ -8,7 +8,7 @@ try
 {
     j = json::parse(my_input);
 }
-catch (json::exception::parse_error& ex)
+catch (json::parse_error& ex)
 {
     std::cerr << "parse error at byte " << ex.byte << std::endl;
 }