The insecure examples code does not include delimiters for strings

Problem/Motivation

The insecure examples shown on FormattableMarkup::placeholderFormat() does not include string delimiters.

$this->placeholderFormat('<@foo>text</@foo>, ['@foo' => $some_variable]);
$this->placeholderFormat('<a @foo>link text</a>, ['@foo' => $some_variable]);
$this->placeholderFormat('<a href="@foo">link text</a>, ['@foo' => $some_variable]);
$this->placeholderFormat('<a title="@foo">link text</a>, ['@foo' => $some_variable]);

Proposed resolution

Change the code to include the final delimiter for those strings. The correct code is the following one.

$this->placeholderFormat('<@foo>text</@foo>', ['@foo' => $some_variable]);
$this->placeholderFormat('<a @foo>link text</a>', ['@foo' => $some_variable]);
$this->placeholderFormat('<a href="@foo">link text</a>', ['@foo' => $some_variable]);
$this->placeholderFormat('<a title="@foo">link text</a>', ['@foo' => $some_variable]);