[wwwdocs] Tweak /gcc-5/porting_to.html wording about converting nullptr to bool

[Jonathan Wakely]

On 10/03/15 18:05 +0000, Jonathan Wakely wrote:
>Since nullptr always converts to false it doesn't make much sense to
>recommend using 'true' instead.
>
>Committed to CVS.
>

Oops, that was only meant to go to gcc-patches, sorry.