[docs] update header in index.rst
[docs] update header in index.rst

- This should stop the top Google link reading 'TLDR'

kwlzn/pex,pombredanne/pex,sixninetynine/pex,pantsbuild/pex,rouge8/pex,snyaggarwal/pex,rouge8/pex,kwlzn/pex,snyaggarwal/pex,URXtech/pex,mzdanieltest/pex,pantsbuild/pex,pombredanne/pex,URXtech/pex,jsirois/pex,jsirois/pex,mzdanieltest/pex,sixninetynine/pex