minor #6557 [ClassLoader] Add missed link to the external PSR-4 specification (nicolas-grekas, fre5h)

This PR was submitted for the 2.6 branch but it was merged into the 2.7 branch instead (closes #6557).

Discussion
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[ClassLoader] Add missed link to the external PSR-4 specification

| Q              | A
| ------------- | ---
| Doc fix?          | yes
| New docs?     | no
| Applies to       | 2.6+
| Fixed tickets   | N/A

There is already the link to the external PSR-0 specification, but for PSR-4 this link is missed. So I added it.

Commits
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9169d7f Add missed link to the external PSR-4 specification.

Author: xabbuh

components/class_loader/introduction.rst

@@ -19,7 +19,7 @@ load your classes:
   the `PSR-0`_ class naming standard;
 
 * :doc:`/components/class_loader/psr4_class_loader`: loads classes that follow
-  the `PSR-4` class naming standard;
+  the `PSR-4`_ class naming standard;
 
 * :doc:`/components/class_loader/map_class_loader`: loads classes using
   a static map from class name to file path.
@@ -45,5 +45,6 @@ You can install the component in 2 different ways:
 .. include:: /components/require_autoload.rst.inc
 
 .. _PSR-0: http://www.php-fig.org/psr/psr-0/
+.. _PSR-4: http://www.php-fig.org/psr/psr-4/
 .. _`autoloading mechanism`: http://php.net/manual/en/language.oop5.autoload.php
 .. _Packagist: https://packagist.org/packages/symfony/class-loader