message: Fix bullets on install page
diff: We've built and tested TensorFlow on the following 64-bit laptop/desktop
  operating systems:
  +
  * MacOS X 10.11 (El Capitan) or later.
  * Ubuntu 14.04 or later
  * Windows 7 or later.
  +
  Although you might be able to install TensorFlow on other laptop or desktop
  systems, we only support (and only fix issues in) the preceding configurations.
  