Fix build URL in new LLDB website

Summary:
After the LLDB website was migrated to be generated by Sphinx the build.html file lives in another location.

See http://lists.llvm.org/pipermail/lldb-dev/2019-April/014992.html.

Reviewers: teemperor

Reviewed By: teemperor

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D61368

llvm-svn: 359665