feat(example): is_vector specialization no longer needed for si_constants

docs/users_guide/examples/si_constants.md

@@ -16,19 +16,11 @@ work in practice.
 --8<-- "example/si_constants.cpp:28:40"
 ```
 
-As always, we start with the inclusion of all the needed header files. After that, for
-the simplicity of this example, we
-[hack the character of quantities](../framework_basics/character_of_a_quantity.md#hacking-the-character)
-to be able to express vector quantities with simple scalar types.
-
-```cpp title="si_constants.cpp" linenums="14"
---8<-- "example/si_constants.cpp:42:44"
-```
-
+As always, we start with the inclusion of all the needed header files.
 The main part of the example prints all of the SI-defining constants:
 
-```cpp title="si_constants.cpp" linenums="17"
---8<-- "example/si_constants.cpp:45:"
+```cpp title="si_constants.cpp" linenums="14"
+--8<-- "example/si_constants.cpp:42:"
 ```
 
 While analyzing the output of this program (provided below), we can easily notice that a direct