minor #13583 [HttpFoundation] minor: clarify Request::getUrlencodedPrefix() regex (dunglas)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #13583).

Discussion
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[HttpFoundation] minor: clarify Request::getUrlencodedPrefix() regex

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR       | n/a

The current notation used is confusing and displays an error in PHPStorm (probably a false positive). This PR clarify the notation.

Commits
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1aba7b4 [HttpFoundation] minor: clarify Request::getUrlencodedPrefix() regex

src/Symfony/Component/HttpFoundation/Request.php

@@ -1847,7 +1847,7 @@ class Request
 
         $len = strlen($prefix);
 
-        if (preg_match("#^(%[[:xdigit:]]{2}|.){{$len}}#", $string, $match)) {
+        if (preg_match(sprintf('#^(%%[[:xdigit:]]{2}|.){%d}#', $len), $string, $match)) {
             return $match[0];
         }