docs - title level inconsistent

[chrisckchang]

We are getting these warnings on build:

carbon-io/docs/packages/carbon-core/docs/packages/atom/docs/index.rst:341: SEVERE: Title level inconsistent:

Argument Parsing
^^^^^^^^^^^^^^^^
/carbon-io/docs/packages/carbon-core/docs/packages/atom/docs/index.rst:374: SEVERE: Title level inconsistent:

Main
^^^^

What title level should these be?

Argument Parsing: https://github.com/carbon-io/atom/blob/master/docs/index.rst#L341
Main: https://github.com/carbon-io/atom/blob/master/docs/index.rst#L374