Fixed the code snippets for the expression language functions

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.4+
| Fixed tickets | n/a

The compilation callable was broken for the example of the lowercase function (in 2 different ways in the different snippet as the first one was attempted to be fixed in #3771 but my comment asking to remove the return statement inside the compiled expression was misunderstood and the other return statement returning the compiled expression was removed as well.).