Updating link to build directory example

So it looks like [OCMock moved from build scripts to a Makefile](https://github.com/erikdoe/ocmock/commit/15b251147255180559d121dc1a1c04f27a2123e8), so the link to the custom build location example broke, since it linked to `master`. In lieu of having to grep through their Makefile to figure out how they use build directories, just link to the absolute reference to their old build script!

:octocat: