[NVPTX] Add blank line to NVPTXUsage.rst to appease the Sphinx.

Fixes:

  Warning, treated as error:
  /home/buildbot/llvm-build-dir/llvm-sphinx-docs/llvm/src/docs/NVPTXUsage.rst:333:
  ERROR: Error in "code-block" directive:
  maximum 1 argument(s) allowed, 17 supplied.

llvm-svn: 292148