Fix typo in regex.md doc page (#7363)

The Regex Features page has a typo
https://docs.rapids.ai/api/libcudf/stable/md_regex.html

The last feature in the **Shorthand Character Classes** section should be `\S` instead of `\s`

Authors:
  - David (@davidwendt)

Approvers:
  - Christopher Harris (@cwharris)
  - Mike Wilson (@hyperbolic2346)

URL: #7363

cpp/doxygen/regex.md

@@ -62,7 +62,7 @@ The details are based on features documented at https://www.regular-expressions.
 | Shorthand | `\s` | Adds all whitespace to the character class. Matches a single whitespace character if used outside character classes. | `\s` matches any single whitespace character |
 | Shorthand | `\D` | Adds all non-digits to the character class. Matches a single character that is not a digit character if used outside character classes. | `[\D]` matches a single character that is not a digit character |
 | Shorthand | `\W` | Adds all non-word characters to the character class. Matches a single character that is not a word character if used outside character classes. | [`\W`] matches a single character that is not a word character |
-| Shorthand | `\s` | Adds all non-whitespace to the character class. Matches a single character that is not a whitespace character if used outside character classes. | `[\S]` matches a single character that is not a whitespace character |
+| Shorthand | `\S` | Adds all non-whitespace to the character class. Matches a single character that is not a whitespace character if used outside character classes. | `[\S]` matches a single character that is not a whitespace character |
 
 ### Anchors