docs: Fix the examples for char_length() and character_length() (#18808)

## Which issue does this PR close?

- Closes N/A.

## Rationale for this change

The examples for `char_length()` and `character_length()` are swapped at
https://datafusion.apache.org/user-guide/expressions.html#string-expressions

## What changes are included in this PR?

Swap the examples to fix the issue.

## Are these changes tested?

No. Only docs are touched.

## Are there any user-facing changes?

No

Signed-off-by: Martin Tzvetanov Grigorov <[email protected]>

Author: martin-g

docs/source/user-guide/expressions.md

@@ -179,8 +179,8 @@ select log(-1), log(0), sqrt(-1);
 | ascii(character)                               | Returns a numeric representation of the character (`character`). Example: `ascii('a') -> 97`                                                                                                                                             |
 | bit_length(text)                               | Returns the length of the string (`text`) in bits. Example: `bit_length('spider') -> 48`                                                                                                                                                 |
 | btrim(text, characters)                        | Removes all specified characters (`characters`) from both the beginning and the end of the string (`text`). Example: `btrim('aabchelloccb', 'abc') -> hello`                                                                             |
-| char_length(text)                              | Returns number of characters in the string (`text`). The same as `character_length` and `length`. Example: `character_length('lion') -> 4`                                                                                               |
-| character_length(text)                         | Returns number of characters in the string (`text`). The same as `char_length` and `length`. Example: `char_length('lion') -> 4`                                                                                                         |
+| char_length(text)                              | Returns number of characters in the string (`text`). The same as `character_length` and `length`. Example: `char_length('lion') -> 4`                                                                                                    |
+| character_length(text)                         | Returns number of characters in the string (`text`). The same as `char_length` and `length`. Example: `character_length('lion') -> 4`                                                                                                    |
 | concat(value1, [value2 [, ...]])               | Concatenates the text representations (`value1, [value2 [, ...]]`) of all the arguments. NULL arguments are ignored. Example: `concat('aaa', 'bbc', NULL, 321) -> aaabbc321`                                                             |
 | concat_ws(separator, value1, [value2 [, ...]]) | Concatenates the text representations (`value1, [value2 [, ...]]`) of all the arguments with the separator (`separator`). NULL arguments are ignored. `concat_ws('/', 'path', 'to', NULL, 'my', 'folder', 123) -> path/to/my/folder/123` |
 | chr(integer)                                   | Returns a character by its numeric representation (`integer`). Example: `chr(90) -> 8`                                                                                                                                                   |