Capture exceptions by const& in docs. (#4099)

docs/examples/object.cpp

@@ -21,7 +21,7 @@ int main()
         // can only create an object from a list of pairs
         json j_invalid_object = json::object({{ "one", 1, 2 }});
     }
-    catch (json::type_error& e)
+    catch (const json::type_error& e)
     {
         std::cout << e.what() << '\n';
     }