http://api.jquery.com/category/selectors/ says:

If you wish to use any of the meta-characters ( such as !"#$%&'()*+,./:;?@[\\]^`{|}~ ) as a literal part of a name, you must escape the character with two backslashes: \\\\. For example, if you have an an element with id="foo.bar", you can use the selector $("#foo\\\\.bar").

Of course, the selector needs a single backslash for escaping. The paragraph is confused because in JavaScript, backslashes also escape string literals, and selectors are usually built from JavaScript string literals. To use the same example, if you have an element with id="foo.bar", you should use the selector "#foo\\.bar", but that selector is written "#foo\\\\.bar" in JavaScript.

The nuance can be tricky, and PHP folks, who have a similar situation with regular expressions, put an explaining note, as can be seen on http://ca.php.net/manual/en/regexp.reference.escape.php

It's quite important to have that note; a simple fix without a similar note would probably be as confusing as the current paragraph is incorrect.

You'll also note that the text contains a double "an" ("an an element").