Fix README headers (#298)

## Type of change

<!--Delete points that do not apply-->

- Bug fix

## Changes

The following changes have been made:

- Makes the Upgradability libraries header an h4 over and h2

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.
- [x] I have updated the changelog to reflect the changes on this PR.

CHANGELOG.md

@@ -22,6 +22,7 @@ Description of the upcoming release here.
 ### Fixed
 
 - [#297](https://github.com/FuelLabs/sway-libs/pull/297) Fixes docs anchor in basic SRC-7 example.
+- [#298](https://github.com/FuelLabs/sway-libs/pull/298) Fixes the README headers on Upgradability Libraries from an `h2` to an `h4`.
 
 #### Breaking
 

README.md

@@ -66,7 +66,7 @@ For implementation details on the libraries please see the [Sway Libs Docs](http
 
 - [Queue](https://docs.fuel.network/docs/sway-libs/queue/) is a linear data structure that provides First-In-First-Out (FIFO) operations.
 
-## Upgradability Libraries
+#### Upgradability Libraries
 
 - [Upgradability](https://docs.fuel.network/docs/sway-libs/upgradability/) provides functions that can be used to implement contract upgrades via simple upgradable proxies.