3.1 PATCH: Fix libstdc++-v3 install.html path

[Rainer Orth]

B. Kosnik writes:

> > Ok for branch and mainline?
> 
> Dude. Let's stop hazing Mark for every single patch.

Oops, sorry I got carried away ;-)

> This is fine for mainline. Queue for 3.1.1

Installed, thanks.

	Rainer