Correct the Bjarne Stroustrup's C++ Page link

Summary: Bjarne Stroustrup's C++ Page link pointing to wrong AT&T page.

Reviewers: jyknight, sanjoy, silvas, hubert.reinterpretcast

Reviewed By: hubert.reinterpretcast

Subscribers: hubert.reinterpretcast, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75709