minor #19418 [Validator] Fix mb_detect_encoding link in Charset (smnandre)

This PR was merged into the 7.1 branch.

Discussion
----------

[Validator] Fix `mb_detect_encoding` link in `Charset`

Fix mb_detect_encoding link sending to 404 ( https://www.php.net/manual/en/function.mb-detect-encoding().php )

Commits
-------

f3ca92f [Validator] Fix mb_detect_encoding link in Charset

Author: javiereguiluz

reference/constraints/Charset.rst

@@ -89,7 +89,7 @@ Options
 An encoding or a set of encodings to check against. If you pass an array of
 encodings, the validator will check if the value is encoded in *any* of the
 encodings. This option accepts any value that can be passed to
-:phpfunction:`mb_detect_encoding()`.
+:phpfunction:`mb_detect_encoding`.
 
 .. include:: /reference/constraints/_groups-option.rst.inc