Made capitalization consistent

[leftmove]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

README edits to create consistency.

Additional context & links

The secondary headers marked with ## were not consistent in capitalization.

Some were uppercase.

## Usage
## Streaming Responses

But others were only partially uppercase, which created some inconsistency.

## Async usage
## Module-level client

I went through and capitalized everything to make it consistent.

Additonally, a header was listed as nested params, but in the description it was referred to as nested parameters, so I removed the short hand, and changed the header to Nested Parameters to create additonal consistency.

[RobertCraigie]

Thanks for raising this @leftmove! I think we're actually going to go with Sentence case, instead of Upper Case, like you've done in this PR so I'm going to close this and make the change on our end.

Thanks for the PR!