Link to the official ES5.1 spec (PDF): change it in order to allow to easily retrieve the HTML version

[claudepache]

In the block at the beginning ("This is not the official ..."), there is a link to the pdf version to the official ECMAScript Language Specification, Edition 5.1 (http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf)

It should be replaced with a link to the following page, where the user can choose between the HTML and the PDF version of it:

http://www.ecma-international.org/publications/standards/Ecma-262.htm