minor #13187 Update filesystem.rst (Yohann76)

This PR was submitted for the 5.0 branch but it was squashed and merged into the 3.4 branch instead (closes #13187).

Discussion
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Update filesystem.rst

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Commits
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e6177f9 Update filesystem.rst

Author: javiereguiluz

components/filesystem.rst

@@ -216,7 +216,7 @@ support symbolic links, a third boolean argument is available::
 
 :method:`Symfony\\Component\\Filesystem\\Filesystem::readlink` read links targets.
 
-PHP's ``readlink()`` function returns the target of a symbolic link. However, its behavior
+PHP's :phpfunction:`readlink` function returns the target of a symbolic link. However, its behavior
 is completely different under Windows and Unix. On Windows systems, ``readlink()``
 resolves recursively the children links of a link until a final target is found. On
 Unix-based systems ``readlink()`` only resolves the next link.