Fix broken link for releases
To build the latest release of OpenEXR, begin by downloading the
source from the GitHub Releases page:
-https://github.com/AcademySoftwareFoundation/releases.
+https://github.com/AcademySoftwareFoundation/openexr/releases.
To build from the latest development version, which may not be stable,
clone the GitHub repo and build from the master branch:
