docs/index.rst: Fix short underline for 'Full list of options'
docs/index.rst: Fix short underline for 'Full list of options'

Avoid:

  .../docs/index.rst:156: WARNING: Title underline too short.

323ec78d (Update documentation of each parameter, 2014-07-29) changed
the title for this section from 'Options' to 'Full list of options',
but failed to adjust the underline length appropriately.

corydolphin/flask-cors,ashleysommer/sanic-cors